diff --git a/.github/workflows/codecov.yml b/.github/workflows/codecov.yml
index 74cd0f9b..88634264 100644
--- a/.github/workflows/codecov.yml
+++ b/.github/workflows/codecov.yml
@@ -12,12 +12,13 @@ jobs:
       CARGO_TERM_COLOR: always
     steps:
       - uses: actions/checkout@v4
-      - name: Install Rust
-        run: rustup update stable
+      # nightly is required for --doctests, see cargo-llvm-cov#2
+      - name: Install Rust (nightly)
+        run: rustup update nightly
       - name: Install cargo-llvm-cov
         uses: taiki-e/install-action@cargo-llvm-cov
       - name: Generate code coverage
-        run: cargo llvm-cov --all-features --workspace --lcov --output-path lcov.info
+        run: cargo +nightly llvm-cov --all-features --workspace --lcov --doctests --output-path lcov.info
       - name: Upload coverage to Codecov
         uses: codecov/codecov-action@v3
         env:
diff --git a/.github/workflows/lint.yml b/.github/workflows/lint.yml
index e8bc1d31..920a702b 100644
--- a/.github/workflows/lint.yml
+++ b/.github/workflows/lint.yml
@@ -17,12 +17,16 @@ jobs:
       - uses: Swatinem/rust-cache@v2
       - run: cargo fmt --check -- --color=always
       - run: cargo fmt --check --manifest-path fuzz/Cargo.toml
+      - run: cargo fmt --check --manifest-path bench/Cargo.toml
       - run: |
           cargo clippy --all-features --all-targets --color=always \
             -- -D warnings
       - run: |
           cargo clippy --manifest-path fuzz/Cargo.toml --color=always \
             -- -D warnings
+      - run: |
+          cargo clippy --manifest-path bench/Cargo.toml --color=always \
+            -- -D warnings
         env:
           RUSTFLAGS: "-Dwarnings"
 
diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 33d00b7a..4479d6ff 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -6,22 +6,10 @@ on:
   pull_request:
 
 jobs:
-  timezones_linux:
+  timezones:
     strategy:
       matrix:
-        os: [ubuntu-latest]
-        tz: ["ACST-9:30", "EST4", "UTC0", "Asia/Katmandu"]
-    runs-on: ${{ matrix.os }}
-    steps:
-      - uses: actions/checkout@v4
-      - uses: dtolnay/rust-toolchain@stable
-      - uses: Swatinem/rust-cache@v2
-      - run: cargo test --all-features --color=always -- --color=always
-
-  timezones_other:
-    strategy:
-      matrix:
-        os: [macos-latest, windows-latest]
+        os: [ubuntu-latest, macos-latest, windows-latest]
         tz: ["ACST-9:30", "EST4", "UTC0", "Asia/Katmandu"]
     runs-on: ${{ matrix.os }}
     steps:
@@ -67,7 +55,7 @@ jobs:
         with:
           toolchain: ${{ matrix.rust_version }}
       - uses: Swatinem/rust-cache@v2
-      - run: cargo check --benches
+      - run: cargo check --manifest-path bench/Cargo.toml --benches
       - run: cargo check --manifest-path fuzz/Cargo.toml --all-targets
       # run --lib and --doc to avoid the long running integration tests
       # which are run elsewhere
@@ -119,7 +107,6 @@ jobs:
         os: [ubuntu-latest]
         target:
           [
-            wasm32-wasi,
             wasm32-unknown-emscripten,
             aarch64-apple-ios,
             aarch64-linux-android,
@@ -145,7 +132,6 @@ jobs:
       - uses: dtolnay/rust-toolchain@stable
         with:
           targets: wasm32-unknown-unknown
-      - uses: taiki-e/install-action@cargo-hack
       - uses: Swatinem/rust-cache@v2
       - uses: actions/setup-node@v3
       - uses: jetli/wasm-pack-action@v0.4.0
@@ -153,6 +139,27 @@ jobs:
       # with the host system.
       - run: TZ="$(date +%z)" NOW="$(date +%s)" wasm-pack test --node -- --features wasmbind
 
+  test_wasi:
+    strategy:
+      matrix:
+        os: [ubuntu-latest]
+        target:
+          - wasm32-wasi
+    runs-on: ${{ matrix.os }}
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@stable
+        with:
+          targets: wasm32-wasi
+      - uses: Swatinem/rust-cache@v2
+      - run: cargo install cargo-wasi
+      - uses: mwilliamson/setup-wasmtime-action@v2
+        with:
+          wasmtime-version: "12.0.1"
+      # We can't use `--all-features` because `rustc-serialize` doesn't support
+      # `wasm32-wasi`.
+      - run: cargo wasi test --features=serde,unstable-locales --color=always -- --color=always
+
   cross-targets:
     strategy:
       matrix:
@@ -165,6 +172,20 @@ jobs:
       - uses: Swatinem/rust-cache@v2
       - run: cross check --target ${{ matrix.target }}
 
+  cross-tests:
+    strategy:
+      matrix:
+        os: [ubuntu-latest]
+    runs-on: ${{ matrix.os }}
+    steps:
+      - uses: actions/checkout@v4
+      - run: cargo install cross
+      - uses: Swatinem/rust-cache@v2
+      - run: cross test --lib --all-features --target i686-unknown-linux-gnu --color=always
+      - run: cross test --doc --all-features --target i686-unknown-linux-gnu --color=always
+      - run: cross test --lib --all-features --target i686-unknown-linux-musl --color=always
+      - run: cross test --doc --all-features --target i686-unknown-linux-musl --color=always
+
   check-docs:
     runs-on: ubuntu-latest
     steps:
diff --git a/CITATION.cff b/CITATION.cff
index e5a1720b..5dd5b1fb 100644
--- a/CITATION.cff
+++ b/CITATION.cff
@@ -3,8 +3,8 @@ cff-version: 1.2.0
 message: Please cite this crate using these information.
 
 # Version information.
-date-released: 2023-08-29
-version: 0.4.27
+date-released: 2023-09-07
+version: 0.4.30
 
 # Project information.
 abstract: Date and time library for Rust
diff --git a/Cargo.toml b/Cargo.toml
index 4cc62c6c..0e24ee01 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -25,13 +25,11 @@ std = []
 clock = ["std", "winapi", "iana-time-zone", "android-tzdata"]
 wasmbind = ["wasm-bindgen", "js-sys"]
 unstable-locales = ["pure-rust-locales", "alloc"]
-__internal_bench = ["criterion"]
-__doctest = []
+__internal_bench = []
 
 [dependencies]
 serde = { version = "1.0.99", default-features = false, optional = true }
 pure-rust-locales = { version = "0.6", optional = true }
-criterion = { version = "0.4.0", optional = true }
 rkyv = { version = "0.7", optional = true }
 arbitrary = { version = "1.0.0", features = ["derive"], optional = true }
 
@@ -55,7 +53,6 @@ android-tzdata = { version = "0.1.1", optional = true }
 serde_json = { version = "1" }
 serde_derive = { version = "1", default-features = false }
 bincode = { version = "1.3.0" }
-doc-comment = { version = "0.3" }
 
 [target.'cfg(all(target_arch = "wasm32", not(any(target_os = "emscripten", target_os = "wasi"))))'.dev-dependencies]
 wasm-bindgen-test = "0.3"
@@ -66,13 +63,3 @@ rustdoc-args = ["--cfg", "docsrs"]
 
 [package.metadata.playground]
 features = ["serde"]
-
-[[bench]]
-name = "chrono"
-required-features = ["__internal_bench"]
-harness = false
-
-[[bench]]
-name = "serde"
-required-features = ["__internal_bench", "serde"]
-harness = false
diff --git a/bench/Cargo.toml b/bench/Cargo.toml
new file mode 100644
index 00000000..4c140688
--- /dev/null
+++ b/bench/Cargo.toml
@@ -0,0 +1,26 @@
+[package]
+name = "benches"
+version = "0.1.0"
+edition = "2021"
+
+# Even as a `dev-dependency` Criterion and its dependencies can affect the MSRV of chrono.
+# But not when it lives in a separate crate :-).
+# See https://github.com/chronotope/chrono/pull/1104.
+
+[lib]
+name = "benches"
+
+[dependencies]
+chrono = { path = "..", features = ["__internal_bench", "serde"] }
+
+[[bench]]
+name = "chrono"
+harness = false
+
+[[bench]]
+name = "serde"
+harness = false
+
+[dev-dependencies]
+criterion = "0.5.0"
+serde_json = "1"
diff --git a/benches/chrono.rs b/bench/benches/chrono.rs
similarity index 99%
rename from benches/chrono.rs
rename to bench/benches/chrono.rs
index 77a446b2..05ec703c 100644
--- a/benches/chrono.rs
+++ b/bench/benches/chrono.rs
@@ -1,5 +1,4 @@
 //! Benchmarks for chrono that just depend on std
-#![cfg(feature = "__internal_bench")]
 
 use criterion::{black_box, criterion_group, criterion_main, BenchmarkId, Criterion};
 
diff --git a/benches/serde.rs b/bench/benches/serde.rs
similarity index 95%
rename from benches/serde.rs
rename to bench/benches/serde.rs
index e9de4408..ee4deb5a 100644
--- a/benches/serde.rs
+++ b/bench/benches/serde.rs
@@ -1,5 +1,3 @@
-#![cfg(feature = "__internal_bench")]
-
 use criterion::{black_box, criterion_group, criterion_main, Criterion};
 
 use chrono::NaiveDateTime;
diff --git a/bench/src/lib.rs b/bench/src/lib.rs
new file mode 100644
index 00000000..d87371ca
--- /dev/null
+++ b/bench/src/lib.rs
@@ -0,0 +1 @@
+// This file only exists to make `benches` a valid crate.
diff --git a/src/datetime/mod.rs b/src/datetime/mod.rs
index cfa045dc..2c6606af 100644
--- a/src/datetime/mod.rs
+++ b/src/datetime/mod.rs
@@ -660,10 +660,24 @@ impl DateTime<FixedOffset> {
     /// and returns a new [`DateTime`] instance with the parsed timezone as the [`FixedOffset`].
     ///
     /// RFC 2822 is the internet message standard that specifies the representation of times in HTTP
-    /// and email headers.
+    /// and email headers. It is the 2001 revision of RFC 822, and is itself revised as RFC 5322 in
+    /// 2008.
     ///
-    /// The RFC 2822 standard allows arbitrary intermixed whitespace.
-    /// See [RFC 2822 Appendix A.5]
+    /// # Support for the obsolete date format
+    ///
+    /// - A 2-digit year is interpreted to be a year in 1950-2049.
+    /// - The standard allows comments and whitespace between many of the tokens. See [4.3] and
+    ///   [Appendix A.5]
+    /// - Single letter 'military' time zone names are parsed as a `-0000` offset.
+    ///   They were defined with the wrong sign in RFC 822 and corrected in RFC 2822. But because
+    ///   the meaning is now ambiguous, the standard says they should be be considered as `-0000`
+    ///   unless there is out-of-band information confirming their meaning.
+    ///   The exception is `Z`, which remains identical to `+0000`.
+    ///
+    /// [4.3]: https://www.rfc-editor.org/rfc/rfc2822#section-4.3
+    /// [Appendix A.5]: https://www.rfc-editor.org/rfc/rfc2822#appendix-A.5
+    ///
+    /// # Example
     ///
     /// ```
     /// # use chrono::{DateTime, FixedOffset, TimeZone};
@@ -672,8 +686,6 @@ impl DateTime<FixedOffset> {
     ///     FixedOffset::east_opt(0).unwrap().with_ymd_and_hms(2015, 2, 18, 23, 16, 9).unwrap()
     /// );
     /// ```
-    ///
-    /// [RFC 2822 Appendix A.5]: https://www.rfc-editor.org/rfc/rfc2822#appendix-A.5
     pub fn parse_from_rfc2822(s: &str) -> ParseResult<DateTime<FixedOffset>> {
         const ITEMS: &[Item<'static>] = &[Item::Fixed(Fixed::RFC2822)];
         let mut parsed = Parsed::new();
diff --git a/src/datetime/tests.rs b/src/datetime/tests.rs
index 88e08952..607f1eeb 100644
--- a/src/datetime/tests.rs
+++ b/src/datetime/tests.rs
@@ -311,6 +311,7 @@ fn ymdhms_nano(
 }
 
 // local helper function to easily create a DateTime<Utc>
+#[cfg(any(feature = "alloc", feature = "std"))]
 fn ymdhms_utc(year: i32, month: u32, day: u32, hour: u32, min: u32, sec: u32) -> DateTime<Utc> {
     Utc.with_ymd_and_hms(year, month, day, hour, min, sec).unwrap()
 }
@@ -818,6 +819,7 @@ fn test_datetime_from_str() {
             .unwrap())
     );
     assert!("2015-2-18T23:16:9.15".parse::<DateTime<Utc>>().is_err());
+    assert!("2015-02-18T23:16:9.15øøø".parse::<DateTime<Utc>>().is_err());
 
     // no test for `DateTime<Local>`, we cannot verify that much.
 }
@@ -915,7 +917,7 @@ fn test_parse_datetime_utc() {
 }
 
 #[test]
-fn test_utc_datetime_from_str() {
+fn test_parse_from_str() {
     let edt = FixedOffset::east_opt(570 * 60).unwrap();
     let edt0 = FixedOffset::east_opt(0).unwrap();
     let wdt = FixedOffset::west_opt(10 * 3600).unwrap();
@@ -930,11 +932,7 @@ fn test_utc_datetime_from_str() {
     )
     .is_err());
     assert_eq!(
-        Utc.datetime_from_str("Fri, 09 Aug 2013 23:54:35 GMT", "%a, %d %b %Y %H:%M:%S GMT"),
-        Ok(Utc.with_ymd_and_hms(2013, 8, 9, 23, 54, 35).unwrap())
-    );
-    assert_eq!(
-        DateTime::<FixedOffset>::parse_from_str("0", "%s").unwrap(),
+        DateTime::<Utc>::parse_from_str("0", "%s").unwrap(),
         NaiveDateTime::from_timestamp_opt(0, 0).unwrap().and_utc().fixed_offset()
     );
 
@@ -978,53 +976,6 @@ fn test_utc_datetime_from_str() {
     // no test for `DateTime<Local>`, we cannot verify that much.
 }
 
-#[test]
-fn test_utc_datetime_from_str_with_spaces() {
-    let dt = ymdhms_utc(2013, 8, 9, 23, 54, 35);
-    // with varying spaces - should succeed
-    assert_eq!(Utc.datetime_from_str(" Aug 09 2013 23:54:35", " %b %d %Y %H:%M:%S"), Ok(dt),);
-    assert_eq!(Utc.datetime_from_str("Aug 09 2013 23:54:35 ", "%b %d %Y %H:%M:%S "), Ok(dt),);
-    assert_eq!(Utc.datetime_from_str(" Aug 09 2013  23:54:35 ", " %b %d %Y  %H:%M:%S "), Ok(dt),);
-    assert_eq!(Utc.datetime_from_str("  Aug 09 2013 23:54:35", "  %b %d %Y %H:%M:%S"), Ok(dt),);
-    assert_eq!(Utc.datetime_from_str("   Aug 09 2013 23:54:35", "   %b %d %Y %H:%M:%S"), Ok(dt),);
-    assert_eq!(
-        Utc.datetime_from_str("\n\tAug 09 2013 23:54:35  ", "\n\t%b %d %Y %H:%M:%S  "),
-        Ok(dt),
-    );
-    assert_eq!(Utc.datetime_from_str("\tAug 09 2013 23:54:35\t", "\t%b %d %Y %H:%M:%S\t"), Ok(dt),);
-    assert_eq!(Utc.datetime_from_str("Aug  09 2013 23:54:35", "%b  %d %Y %H:%M:%S"), Ok(dt),);
-    assert_eq!(Utc.datetime_from_str("Aug    09 2013 23:54:35", "%b    %d %Y %H:%M:%S"), Ok(dt),);
-    assert_eq!(Utc.datetime_from_str("Aug  09 2013\t23:54:35", "%b  %d %Y\t%H:%M:%S"), Ok(dt),);
-    assert_eq!(Utc.datetime_from_str("Aug  09 2013\t\t23:54:35", "%b  %d %Y\t\t%H:%M:%S"), Ok(dt),);
-    // with varying spaces - should fail
-    // leading whitespace in format
-    assert!(Utc.datetime_from_str("Aug 09 2013 23:54:35", " %b %d %Y %H:%M:%S").is_err());
-    // trailing whitespace in format
-    assert!(Utc.datetime_from_str("Aug 09 2013 23:54:35", "%b %d %Y %H:%M:%S ").is_err());
-    // extra mid-string whitespace in format
-    assert!(Utc.datetime_from_str("Aug 09 2013 23:54:35", "%b %d %Y  %H:%M:%S").is_err());
-    // mismatched leading whitespace
-    assert!(Utc.datetime_from_str("\tAug 09 2013 23:54:35", "\n%b %d %Y %H:%M:%S").is_err());
-    // mismatched trailing whitespace
-    assert!(Utc.datetime_from_str("Aug 09 2013 23:54:35 ", "%b %d %Y %H:%M:%S\n").is_err());
-    // mismatched mid-string whitespace
-    assert!(Utc.datetime_from_str("Aug 09 2013 23:54:35", "%b %d %Y\t%H:%M:%S").is_err());
-    // trailing whitespace in format
-    assert!(Utc.datetime_from_str("Aug 09 2013 23:54:35", "%b %d %Y %H:%M:%S ").is_err());
-    // trailing whitespace (newline) in format
-    assert!(Utc.datetime_from_str("Aug 09 2013 23:54:35", "%b %d %Y %H:%M:%S\n").is_err());
-    // leading space in data
-    assert!(Utc.datetime_from_str(" Aug 09 2013 23:54:35", "%b %d %Y %H:%M:%S").is_err());
-    // trailing space in data
-    assert!(Utc.datetime_from_str("Aug 09 2013 23:54:35 ", "%b %d %Y %H:%M:%S").is_err());
-    // trailing tab in data
-    assert!(Utc.datetime_from_str("Aug 09 2013 23:54:35\t", "%b %d %Y %H:%M:%S").is_err());
-    // mismatched newlines
-    assert!(Utc.datetime_from_str("\nAug 09 2013 23:54:35", "%b %d %Y %H:%M:%S\n").is_err());
-    // trailing literal in data
-    assert!(Utc.datetime_from_str("Aug 09 2013 23:54:35 !!!", "%b %d %Y %H:%M:%S ").is_err());
-}
-
 #[test]
 fn test_datetime_parse_from_str() {
     let dt = ymdhms(&FixedOffset::east_opt(-9 * 60 * 60).unwrap(), 2013, 8, 9, 23, 54, 35);
@@ -1218,8 +1169,11 @@ fn test_subsecond_part() {
     assert_eq!(1234567, datetime.timestamp_subsec_nanos());
 }
 
+// Some targets, such as `wasm32-wasi`, have a problematic definition of `SystemTime`, such as an
+// `i32` (year 2035 problem), or an `u64` (no values before `UNIX-EPOCH`).
+// See https://github.com/rust-lang/rust/issues/44394.
 #[test]
-#[cfg(feature = "std")]
+#[cfg(all(feature = "std", not(all(target_arch = "wasm32", target_os = "wasi"))))]
 fn test_from_system_time() {
     use std::time::{Duration, SystemTime, UNIX_EPOCH};
 
diff --git a/src/format/mod.rs b/src/format/mod.rs
index d4eac190..8940b074 100644
--- a/src/format/mod.rs
+++ b/src/format/mod.rs
@@ -18,14 +18,14 @@
 //! # Example
 #![cfg_attr(not(feature = "std"), doc = "```ignore")]
 #![cfg_attr(feature = "std", doc = "```rust")]
-//! use chrono::{TimeZone, Utc};
+//! use chrono::{NaiveDateTime, TimeZone, Utc};
 //!
 //! let date_time = Utc.with_ymd_and_hms(2020, 11, 10, 0, 1, 32).unwrap();
 //!
 //! let formatted = format!("{}", date_time.format("%Y-%m-%d %H:%M:%S"));
 //! assert_eq!(formatted, "2020-11-10 00:01:32");
 //!
-//! let parsed = Utc.datetime_from_str(&formatted, "%Y-%m-%d %H:%M:%S")?;
+//! let parsed = NaiveDateTime::parse_from_str(&formatted, "%Y-%m-%d %H:%M:%S")?.and_utc();
 //! assert_eq!(parsed, date_time);
 //! # Ok::<(), chrono::ParseError>(())
 //! ```
@@ -451,7 +451,7 @@ const BAD_FORMAT: ParseError = ParseError(ParseErrorKind::BadFormat);
 
 // this implementation is here only because we need some private code from `scan`
 
-/// Parsing a `str` into a `Weekday` uses the format [`%W`](./format/strftime/index.html).
+/// Parsing a `str` into a `Weekday` uses the format [`%A`](./format/strftime/index.html).
 ///
 /// # Example
 ///
@@ -487,7 +487,7 @@ impl FromStr for Weekday {
     }
 }
 
-/// Parsing a `str` into a `Month` uses the format [`%W`](./format/strftime/index.html).
+/// Parsing a `str` into a `Month` uses the format [`%B`](./format/strftime/index.html).
 ///
 /// # Example
 ///
diff --git a/src/format/parse.rs b/src/format/parse.rs
index c325e314..6d49cc09 100644
--- a/src/format/parse.rs
+++ b/src/format/parse.rs
@@ -614,7 +614,7 @@ fn parse_rfc3339_relaxed<'a>(parsed: &mut Parsed, mut s: &'a str) -> ParseResult
         Ok(_) => return Err(NOT_ENOUGH),
     };
     s = s.trim_start();
-    let (s, offset) = if s.len() >= 3 && "UTC".eq_ignore_ascii_case(&s[..3]) {
+    let (s, offset) = if s.len() >= 3 && "UTC".as_bytes().eq_ignore_ascii_case(&s.as_bytes()[..3]) {
         (&s[3..], 0)
     } else {
         scan::timezone_offset(s, scan::consume_colon_maybe, true, false, true)?
@@ -626,7 +626,7 @@ fn parse_rfc3339_relaxed<'a>(parsed: &mut Parsed, mut s: &'a str) -> ParseResult
 #[cfg(test)]
 mod tests {
     use crate::format::*;
-    use crate::{DateTime, FixedOffset, TimeZone, Timelike, Utc};
+    use crate::{DateTime, FixedOffset, NaiveDateTime, TimeZone, Timelike, Utc};
 
     macro_rules! parsed {
         ($($k:ident: $v:expr),*) => (#[allow(unused_mut)] {
@@ -1733,7 +1733,10 @@ mod tests {
         assert_eq!(dt.format(RFC850_FMT).to_string(), "Sunday, 06-Nov-94 08:49:37 GMT");
 
         // Check that it parses correctly
-        assert_eq!(Ok(dt), Utc.datetime_from_str("Sunday, 06-Nov-94 08:49:37 GMT", RFC850_FMT));
+        assert_eq!(
+            NaiveDateTime::parse_from_str("Sunday, 06-Nov-94 08:49:37 GMT", RFC850_FMT),
+            Ok(dt.naive_utc())
+        );
 
         // Check that the rest of the weekdays parse correctly (this test originally failed because
         // Sunday parsed incorrectly).
@@ -1765,7 +1768,7 @@ mod tests {
         ];
 
         for val in &testdates {
-            assert_eq!(Ok(val.0), Utc.datetime_from_str(val.1, RFC850_FMT));
+            assert_eq!(NaiveDateTime::parse_from_str(val.1, RFC850_FMT), Ok(val.0.naive_utc()));
         }
 
         let test_dates_fail = [
@@ -1784,7 +1787,7 @@ mod tests {
         ];
 
         for val in &test_dates_fail {
-            assert!(Utc.datetime_from_str(val, RFC850_FMT).is_err());
+            assert!(NaiveDateTime::parse_from_str(val, RFC850_FMT).is_err());
         }
     }
 
diff --git a/src/format/strftime.rs b/src/format/strftime.rs
index 63c0427c..41e53b3f 100644
--- a/src/format/strftime.rs
+++ b/src/format/strftime.rs
@@ -909,11 +909,20 @@ mod tests {
     }
 
     #[test]
-    #[cfg(feature = "unstable-locales")]
+    #[cfg(all(feature = "unstable-locales", target_pointer_width = "64"))]
     fn test_type_sizes() {
         use core::mem::size_of;
         assert_eq!(size_of::<Item>(), 24);
         assert_eq!(size_of::<StrftimeItems>(), 56);
         assert_eq!(size_of::<Locale>(), 2);
     }
+
+    #[test]
+    #[cfg(all(feature = "unstable-locales", target_pointer_width = "32"))]
+    fn test_type_sizes() {
+        use core::mem::size_of;
+        assert_eq!(size_of::<Item>(), 12);
+        assert_eq!(size_of::<StrftimeItems>(), 28);
+        assert_eq!(size_of::<Locale>(), 2);
+    }
 }
diff --git a/src/lib.rs b/src/lib.rs
index ff6641eb..89d8fca5 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,20 +1,21 @@
 //! # Chrono: Date and Time for Rust
 //!
-//! It aims to be a feature-complete superset of
-//! the [time](https://github.com/rust-lang-deprecated/time) library.
-//! In particular,
+
+//! Chrono aims to provide all functionality needed to do correct operations on dates and times in the
+//! [proleptic Gregorian calendar](https://en.wikipedia.org/wiki/Proleptic_Gregorian_calendar):
 //!
-//! * Chrono strictly adheres to ISO 8601.
-//! * Chrono is timezone-aware by default, with separate timezone-naive types.
-//! * Chrono is space-optimal and (while not being the primary goal) reasonably efficient.
+//! * The [`DateTime`](https://docs.rs/chrono/latest/chrono/struct.DateTime.html) type is timezone-aware
+//!   by default, with separate timezone-naive types.
+//! * Operations that may produce an invalid or ambiguous date and time return `Option` or
+//!   [`LocalResult`](https://docs.rs/chrono/latest/chrono/offset/enum.LocalResult.html).
+//! * Configurable parsing and formatting with a `strftime` inspired date and time formatting syntax.
+//! * The [`Local`](https://docs.rs/chrono/latest/chrono/offset/struct.Local.html) timezone works with
+//!   the current timezone of the OS.
+//! * Types and operations are implemented to be reasonably efficient.
 //!
-//! There were several previous attempts to bring a good date and time library to Rust,
-//! which Chrono builds upon and should acknowledge:
-//!
-//! * [Initial research on
-//!    the wiki](https://github.com/rust-lang/rust-wiki-backup/blob/master/Lib-datetime.md)
-//! * Dietrich Epp's [datetime-rs](https://github.com/depp/datetime-rs)
-//! * Luis de Bethencourt's [rust-datetime](https://github.com/luisbg/rust-datetime)
+//! Timezone data is not shipped with chrono by default to limit binary sizes. Use the companion crate
+//! [Chrono-TZ](https://crates.io/crates/chrono-tz) or [`tzfile`](https://crates.io/crates/tzfile) for
+//! full timezone support.
 //!
 //! ### Features
 //!
@@ -29,10 +30,13 @@
 //!   and traits.
 //! - `clock`: Enables reading the system time (`now`) that depends on the standard library for
 //! UNIX-like operating systems and the Windows API (`winapi`) for Windows.
+//! - `wasmbind`: Interface with the JS Date API for the `wasm32` target.
 //!
 //! Optional features:
 //!
 //! - [`serde`][]: Enable serialization/deserialization via serde.
+//! - `rkyv`: Enable serialization/deserialization via rkyv.
+//! - `arbitrary`: construct arbitrary instances of a type with the Arbitrary crate.
 //! - `unstable-locales`: Enable localization. This adds various methods with a
 //!   `_localized` suffix. The implementation and API may change or even be
 //!   removed in a patch release. Feedback welcome.
@@ -284,16 +288,12 @@
 //!            Ok(fixed_dt.clone()));
 //! assert_eq!(DateTime::<FixedOffset>::parse_from_rfc3339("2014-11-28T21:00:09+09:00"), Ok(fixed_dt.clone()));
 //!
-//! // method 3
-//! assert_eq!(Utc.datetime_from_str("2014-11-28 12:00:09", "%Y-%m-%d %H:%M:%S"), Ok(dt.clone()));
-//! assert_eq!(Utc.datetime_from_str("Fri Nov 28 12:00:09 2014", "%a %b %e %T %Y"), Ok(dt.clone()));
-//!
 //! // oops, the year is missing!
-//! assert!(Utc.datetime_from_str("Fri Nov 28 12:00:09", "%a %b %e %T %Y").is_err());
+//! assert!(DateTime::<FixedOffset>::parse_from_str("Fri Nov 28 12:00:09", "%a %b %e %T %Y").is_err());
 //! // oops, the format string does not include the year at all!
-//! assert!(Utc.datetime_from_str("Fri Nov 28 12:00:09", "%a %b %e %T").is_err());
+//! assert!(DateTime::<FixedOffset>::parse_from_str("Fri Nov 28 12:00:09", "%a %b %e %T").is_err());
 //! // oops, the weekday is incorrect!
-//! assert!(Utc.datetime_from_str("Sat Nov 28 12:00:09 2014", "%a %b %e %T %Y").is_err());
+//! assert!(DateTime::<FixedOffset>::parse_from_str("Sat Nov 28 12:00:09 2014", "%a %b %e %T %Y").is_err());
 //! ```
 //!
 //! Again : See [`format::strftime`](./format/strftime/index.html#specifiers)
@@ -343,18 +343,18 @@
 //!
 //! ## Limitations
 //!
-//! Only proleptic Gregorian calendar (i.e. extended to support older dates) is supported.
-//! Be very careful if you really have to deal with pre-20C dates, they can be in Julian or others.
+//! Only the proleptic Gregorian calendar (i.e. extended to support older dates) is supported.
+//! Date types are limited to about +/- 262,000 years from the common epoch.
+//! Time types are limited to nanosecond accuracy.
+//! Leap seconds can be represented, but Chrono does not fully support them.
+//! See [Leap Second Handling](https://docs.rs/chrono/latest/chrono/naive/struct.NaiveTime.html#leap-second-handling).
 //!
-//! Date types are limited in about +/- 262,000 years from the common epoch.
-//! Time types are limited in the nanosecond accuracy.
+//! ## Rust version requirements
 //!
-//! [Leap seconds are supported in the representation but
-//! Chrono doesn't try to make use of them](./naive/struct.NaiveTime.html#leap-second-handling).
-//! (The main reason is that leap seconds are not really predictable.)
-//! Almost *every* operation over the possible leap seconds will ignore them.
-//! Consider using `NaiveDateTime` with the implicit TAI (International Atomic Time) scale
-//! if you want.
+//! The Minimum Supported Rust Version (MSRV) is currently **Rust 1.57.0**.
+//!
+//! The MSRV is explicitly tested in CI. It may be bumped in minor releases, but this is not done
+//! lightly.
 //!
 //! Chrono inherently does not support an inaccurate or partial date and time representation.
 //! Any operation that can be ambiguous will return `None` in such cases.
@@ -367,6 +367,89 @@
 //!
 //! Advanced time zone handling is not yet supported.
 //! For now you can try the [Chrono-tz](https://github.com/chronotope/chrono-tz/) crate instead.
+//!
+//! ## Relation between chrono and time 0.1
+//!
+//! Rust first had a `time` module added to `std` in its 0.7 release. It later moved to
+//! `libextra`, and then to a `libtime` library shipped alongside the standard library. In 2014
+//! work on chrono started in order to provide a full-featured date and time library in Rust.
+//! Some improvements from chrono made it into the standard library; notably, `chrono::Duration`
+//! was included as `std::time::Duration` ([rust#15934]) in 2014.
+//!
+//! In preparation of Rust 1.0 at the end of 2014 `libtime` was moved out of the Rust distro and
+//! into the `time` crate to eventually be redesigned ([rust#18832], [rust#18858]), like the
+//! `num` and `rand` crates. Of course chrono kept its dependency on this `time` crate. `time`
+//! started re-exporting `std::time::Duration` during this period. Later, the standard library was
+//! changed to have a more limited unsigned `Duration` type ([rust#24920], [RFC 1040]), while the
+//! `time` crate kept the full functionality with `time::Duration`. `time::Duration` had been a
+//! part of chrono's public API.
+//!
+//! By 2016 `time` 0.1 lived under the `rust-lang-deprecated` organisation and was not actively
+//! maintained ([time#136]). chrono absorbed the platform functionality and `Duration` type of the
+//! `time` crate in [chrono#478] (the work started in [chrono#286]). In order to preserve
+//! compatibility with downstream crates depending on `time` and `chrono` sharing a `Duration`
+//! type, chrono kept depending on time 0.1. chrono offered the option to opt out of the `time`
+//! dependency by disabling the `oldtime` feature (swapping it out for an effectively similar
+//! chrono type). In 2019, @jhpratt took over maintenance on the `time` crate and released what
+//! amounts to a new crate as `time` 0.2.
+//!
+//! [rust#15934]: https://github.com/rust-lang/rust/pull/15934
+//! [rust#18832]: https://github.com/rust-lang/rust/pull/18832#issuecomment-62448221
+//! [rust#18858]: https://github.com/rust-lang/rust/pull/18858
+//! [rust#24920]: https://github.com/rust-lang/rust/pull/24920
+//! [RFC 1040]: https://rust-lang.github.io/rfcs/1040-duration-reform.html
+//! [time#136]: https://github.com/time-rs/time/issues/136
+//! [chrono#286]: https://github.com/chronotope/chrono/pull/286
+//! [chrono#478]: https://github.com/chronotope/chrono/pull/478
+//!
+//! ## Security advisories
+//!
+//! In November of 2020 [CVE-2020-26235] and [RUSTSEC-2020-0071] were opened against the `time` crate.
+//! @quininer had found that calls to `localtime_r` may be unsound ([chrono#499]). Eventually, almost
+//! a year later, this was also made into a security advisory against chrono as [RUSTSEC-2020-0159],
+//! which had platform code similar to `time`.
+//!
+//! On Unix-like systems a process is given a timezone id or description via the `TZ` environment
+//! variable. We need this timezone data to calculate the current local time from a value that is
+//! in UTC, such as the time from the system clock. `time` 0.1 and chrono used the POSIX function
+//! `localtime_r` to do the conversion to local time, which reads the `TZ` variable.
+//!
+//! Rust assumes the environment to be writable and uses locks to access it from multiple threads.
+//! Some other programming languages and libraries use similar locking strategies, but these are
+//! typically not shared across languages. More importantly, POSIX declares modifying the
+//! environment in a multi-threaded process as unsafe, and `getenv` in libc can't be changed to
+//! take a lock because it returns a pointer to the data (see [rust#27970] for more discussion).
+//!
+//! Since version 4.20 chrono no longer uses `localtime_r`, instead using Rust code to query the
+//! timezone (from the `TZ` variable or via `iana-time-zone` as a fallback) and work with data
+//! from the system timezone database directly. The code for this was forked from the [tz-rs crate]
+//! by @x-hgg-x. As such, chrono now respects the Rust lock when reading the `TZ` environment
+//! variable. In general, code should avoid modifying the environment.
+//!
+//! [CVE-2020-26235]: https://nvd.nist.gov/vuln/detail/CVE-2020-26235
+//! [RUSTSEC-2020-0071]: https://rustsec.org/advisories/RUSTSEC-2020-0071
+//! [chrono#499]: https://github.com/chronotope/chrono/pull/499
+//! [RUSTSEC-2020-0159]: https://rustsec.org/advisories/RUSTSEC-2020-0159.html
+//! [rust#27970]: https://github.com/rust-lang/rust/issues/27970
+//! [chrono#677]: https://github.com/chronotope/chrono/pull/677
+//! [tz-rs crate]: https://crates.io/crates/tz-rs
+//!
+//! ## Removing time 0.1
+//!
+//! Because time 0.1 has been unmaintained for years, however, the security advisory mentioned
+//! above has not been addressed. While chrono maintainers were careful not to break backwards
+//! compatibility with the `time::Duration` type, there has been a long stream of issues from
+//! users inquiring about the time 0.1 dependency with the vulnerability. We investigated the
+//! potential breakage of removing the time 0.1 dependency in [chrono#1095] using a crater-like
+//! experiment and determined that the potential for breaking (public) dependencies is very low.
+//! We reached out to those few crates that did still depend on compatibility with time 0.1.
+//!
+//! As such, for chrono 0.4.30 we have decided to swap out the time 0.1 `Duration` implementation
+//! for a local one that will offer a strict superset of the existing API going forward. This
+//! will prevent most downstream users from being affected by the security vulnerability in time
+//! 0.1 while minimizing the ecosystem impact of semver-incompatible version churn.
+//!
+//! [chrono#1095]: https://github.com/chronotope/chrono/pull/1095
 
 #![doc(html_root_url = "https://docs.rs/chrono/latest/", test(attr(deny(warnings))))]
 #![cfg_attr(feature = "bench", feature(test))] // lib stability features as per RFC #507
@@ -385,14 +468,6 @@ use core::fmt;
 mod time_delta;
 pub use time_delta::TimeDelta;
 
-#[cfg(feature = "__doctest")]
-#[cfg_attr(feature = "__doctest", cfg(doctest))]
-use doc_comment::doctest;
-
-#[cfg(feature = "__doctest")]
-#[cfg_attr(feature = "__doctest", cfg(doctest))]
-doctest!("../README.md");
-
 /// A convenience module appropriate for glob imports (`use chrono::prelude::*;`).
 pub mod prelude {
     #[doc(no_inline)]
diff --git a/src/naive/date.rs b/src/naive/date.rs
index 2aef693f..954baa91 100644
--- a/src/naive/date.rs
+++ b/src/naive/date.rs
@@ -1409,6 +1409,21 @@ impl NaiveDate {
         NaiveWeek { date: *self, start }
     }
 
+    /// Returns `true` if this is a leap year.
+    ///
+    /// ```
+    /// # use chrono::NaiveDate;
+    /// assert_eq!(NaiveDate::from_ymd_opt(2000, 1, 1).unwrap().leap_year(), true);
+    /// assert_eq!(NaiveDate::from_ymd_opt(2001, 1, 1).unwrap().leap_year(), false);
+    /// assert_eq!(NaiveDate::from_ymd_opt(2002, 1, 1).unwrap().leap_year(), false);
+    /// assert_eq!(NaiveDate::from_ymd_opt(2003, 1, 1).unwrap().leap_year(), false);
+    /// assert_eq!(NaiveDate::from_ymd_opt(2004, 1, 1).unwrap().leap_year(), true);
+    /// assert_eq!(NaiveDate::from_ymd_opt(2100, 1, 1).unwrap().leap_year(), false);
+    /// ```
+    pub const fn leap_year(&self) -> bool {
+        self.ymdf & (0b1000) == 0
+    }
+
     // This duplicates `Datelike::year()`, because trait methods can't be const yet.
     #[inline]
     const fn year(&self) -> i32 {
@@ -3172,6 +3187,16 @@ mod tests {
         assert!(dt.with_ordinal0(4294967295).is_none());
     }
 
+    #[test]
+    fn test_leap_year() {
+        for year in 0..=MAX_YEAR {
+            let date = NaiveDate::from_ymd_opt(year, 1, 1).unwrap();
+            let is_leap = year % 4 == 0 && (year % 100 != 0 || year % 400 == 0);
+            assert_eq!(date.leap_year(), is_leap);
+            assert_eq!(date.leap_year(), date.with_ordinal(366).is_some());
+        }
+    }
+
     //   MAX_YEAR-12-31 minus 0000-01-01
     // = ((MAX_YEAR+1)-01-01 minus 0001-01-01) + (0001-01-01 minus 0000-01-01) - 1 day
     // = ((MAX_YEAR+1)-01-01 minus 0001-01-01) + 365 days
diff --git a/src/naive/datetime/tests.rs b/src/naive/datetime/tests.rs
index c1ee385d..91370464 100644
--- a/src/naive/datetime/tests.rs
+++ b/src/naive/datetime/tests.rs
@@ -348,6 +348,51 @@ fn test_datetime_parse_from_str() {
     );
 }
 
+#[test]
+fn test_datetime_parse_from_str_with_spaces() {
+    let parse_from_str = NaiveDateTime::parse_from_str;
+    let dt = NaiveDate::from_ymd_opt(2013, 8, 9).unwrap().and_hms_opt(23, 54, 35).unwrap();
+    // with varying spaces - should succeed
+    assert_eq!(parse_from_str(" Aug 09 2013 23:54:35", " %b %d %Y %H:%M:%S"), Ok(dt));
+    assert_eq!(parse_from_str("Aug 09 2013 23:54:35 ", "%b %d %Y %H:%M:%S "), Ok(dt));
+    assert_eq!(parse_from_str(" Aug 09 2013  23:54:35 ", " %b %d %Y  %H:%M:%S "), Ok(dt));
+    assert_eq!(parse_from_str("  Aug 09 2013 23:54:35", "  %b %d %Y %H:%M:%S"), Ok(dt));
+    assert_eq!(parse_from_str("   Aug 09 2013 23:54:35", "   %b %d %Y %H:%M:%S"), Ok(dt));
+    assert_eq!(parse_from_str("\n\tAug 09 2013 23:54:35  ", "\n\t%b %d %Y %H:%M:%S  "), Ok(dt),);
+    assert_eq!(parse_from_str("\tAug 09 2013 23:54:35\t", "\t%b %d %Y %H:%M:%S\t"), Ok(dt));
+    assert_eq!(parse_from_str("Aug  09 2013 23:54:35", "%b  %d %Y %H:%M:%S"), Ok(dt));
+    assert_eq!(parse_from_str("Aug    09 2013 23:54:35", "%b    %d %Y %H:%M:%S"), Ok(dt));
+    assert_eq!(parse_from_str("Aug  09 2013\t23:54:35", "%b  %d %Y\t%H:%M:%S"), Ok(dt));
+    assert_eq!(parse_from_str("Aug  09 2013\t\t23:54:35", "%b  %d %Y\t\t%H:%M:%S"), Ok(dt));
+    // with varying spaces - should fail
+    // leading whitespace in format
+    assert!(parse_from_str("Aug 09 2013 23:54:35", " %b %d %Y %H:%M:%S").is_err());
+    // trailing whitespace in format
+    assert!(parse_from_str("Aug 09 2013 23:54:35", "%b %d %Y %H:%M:%S ").is_err());
+    // extra mid-string whitespace in format
+    assert!(parse_from_str("Aug 09 2013 23:54:35", "%b %d %Y  %H:%M:%S").is_err());
+    // mismatched leading whitespace
+    assert!(parse_from_str("\tAug 09 2013 23:54:35", "\n%b %d %Y %H:%M:%S").is_err());
+    // mismatched trailing whitespace
+    assert!(parse_from_str("Aug 09 2013 23:54:35 ", "%b %d %Y %H:%M:%S\n").is_err());
+    // mismatched mid-string whitespace
+    assert!(parse_from_str("Aug 09 2013 23:54:35", "%b %d %Y\t%H:%M:%S").is_err());
+    // trailing whitespace in format
+    assert!(parse_from_str("Aug 09 2013 23:54:35", "%b %d %Y %H:%M:%S ").is_err());
+    // trailing whitespace (newline) in format
+    assert!(parse_from_str("Aug 09 2013 23:54:35", "%b %d %Y %H:%M:%S\n").is_err());
+    // leading space in data
+    assert!(parse_from_str(" Aug 09 2013 23:54:35", "%b %d %Y %H:%M:%S").is_err());
+    // trailing space in data
+    assert!(parse_from_str("Aug 09 2013 23:54:35 ", "%b %d %Y %H:%M:%S").is_err());
+    // trailing tab in data
+    assert!(parse_from_str("Aug 09 2013 23:54:35\t", "%b %d %Y %H:%M:%S").is_err());
+    // mismatched newlines
+    assert!(parse_from_str("\nAug 09 2013 23:54:35", "%b %d %Y %H:%M:%S\n").is_err());
+    // trailing literal in data
+    assert!(parse_from_str("Aug 09 2013 23:54:35 !!!", "%b %d %Y %H:%M:%S ").is_err());
+}
+
 #[test]
 fn test_datetime_add_sub_invariant() {
     // issue #37
diff --git a/src/offset/local/mod.rs b/src/offset/local/mod.rs
index d7e7792c..601b92d7 100644
--- a/src/offset/local/mod.rs
+++ b/src/offset/local/mod.rs
@@ -257,12 +257,4 @@ mod tests {
             );
         }
     }
-
-    /// Test Issue #866
-    #[test]
-    fn test_issue_866() {
-        #[allow(deprecated)]
-        let local_20221106 = Local.ymd(2022, 11, 6);
-        let _dt_20221106 = local_20221106.and_hms_milli_opt(1, 2, 59, 1000).unwrap();
-    }
 }
diff --git a/src/offset/local/tz_info/timezone.rs b/src/offset/local/tz_info/timezone.rs
index 4cf6afe3..02b6f341 100644
--- a/src/offset/local/tz_info/timezone.rs
+++ b/src/offset/local/tz_info/timezone.rs
@@ -859,8 +859,12 @@ mod tests {
                 assert_eq!(time_zone_local, time_zone_local_1);
             }
 
-            let time_zone_utc = TimeZone::from_posix_tz("UTC")?;
-            assert_eq!(time_zone_utc.find_local_time_type(0)?.offset(), 0);
+            // `TimeZone::from_posix_tz("UTC")` will return `Error` if the environment does not have
+            // a time zone database, like for example some docker containers.
+            // In that case skip the test.
+            if let Ok(time_zone_utc) = TimeZone::from_posix_tz("UTC") {
+                assert_eq!(time_zone_utc.find_local_time_type(0)?.offset(), 0);
+            }
         }
 
         assert!(TimeZone::from_posix_tz("EST5EDT,0/0,J365/25").is_err());
diff --git a/src/offset/mod.rs b/src/offset/mod.rs
index ea539153..419550e2 100644
--- a/src/offset/mod.rs
+++ b/src/offset/mod.rs
@@ -444,6 +444,7 @@ pub trait TimeZone: Sized + Clone {
     ///
     /// See also [`DateTime::parse_from_str`] which gives a [`DateTime`] with
     /// parsed [`FixedOffset`].
+    #[deprecated(since = "0.4.29", note = "use `DateTime::parse_from_str` instead")]
     fn datetime_from_str(&self, s: &str, fmt: &str) -> ParseResult<DateTime<Self>> {
         let mut parsed = Parsed::new();
         parse(&mut parsed, s, StrftimeItems::new(fmt))?;
diff --git a/src/time_delta.rs b/src/time_delta.rs
index 90e15f54..0b11be2d 100644
--- a/src/time_delta.rs
+++ b/src/time_delta.rs
@@ -34,9 +34,9 @@ const SECS_PER_MINUTE: i64 = 60;
 /// The number of seconds in an hour.
 const SECS_PER_HOUR: i64 = 3600;
 /// The number of (non-leap) seconds in days.
-const SECS_PER_DAY: i64 = 86400;
+const SECS_PER_DAY: i64 = 86_400;
 /// The number of (non-leap) seconds in a week.
-const SECS_PER_WEEK: i64 = 604800;
+const SECS_PER_WEEK: i64 = 604_800;
 
 macro_rules! try_opt {
     ($e:expr) => {
@@ -492,19 +492,22 @@ mod tests {
         assert!(TimeDelta::seconds(1) != TimeDelta::zero());
         assert_eq!(TimeDelta::seconds(1) + TimeDelta::seconds(2), TimeDelta::seconds(3));
         assert_eq!(
-            TimeDelta::seconds(86399) + TimeDelta::seconds(4),
+            TimeDelta::seconds(86_399) + TimeDelta::seconds(4),
             TimeDelta::days(1) + TimeDelta::seconds(3)
         );
-        assert_eq!(TimeDelta::days(10) - TimeDelta::seconds(1000), TimeDelta::seconds(863000));
-        assert_eq!(TimeDelta::days(10) - TimeDelta::seconds(1000000), TimeDelta::seconds(-136000));
+        assert_eq!(TimeDelta::days(10) - TimeDelta::seconds(1000), TimeDelta::seconds(863_000));
         assert_eq!(
-            TimeDelta::days(2) + TimeDelta::seconds(86399) + TimeDelta::nanoseconds(1234567890),
-            TimeDelta::days(3) + TimeDelta::nanoseconds(234567890)
+            TimeDelta::days(10) - TimeDelta::seconds(1_000_000),
+            TimeDelta::seconds(-136_000)
+        );
+        assert_eq!(
+            TimeDelta::days(2) + TimeDelta::seconds(86_399) + TimeDelta::nanoseconds(1_234_567_890),
+            TimeDelta::days(3) + TimeDelta::nanoseconds(234_567_890)
         );
         assert_eq!(-TimeDelta::days(3), TimeDelta::days(-3));
         assert_eq!(
             -(TimeDelta::days(3) + TimeDelta::seconds(70)),
-            TimeDelta::days(-4) + TimeDelta::seconds(86400 - 70)
+            TimeDelta::days(-4) + TimeDelta::seconds(86_400 - 70)
         );
     }
 
@@ -513,10 +516,10 @@ mod tests {
         assert_eq!(TimeDelta::zero().num_days(), 0);
         assert_eq!(TimeDelta::days(1).num_days(), 1);
         assert_eq!(TimeDelta::days(-1).num_days(), -1);
-        assert_eq!(TimeDelta::seconds(86399).num_days(), 0);
-        assert_eq!(TimeDelta::seconds(86401).num_days(), 1);
-        assert_eq!(TimeDelta::seconds(-86399).num_days(), 0);
-        assert_eq!(TimeDelta::seconds(-86401).num_days(), -1);
+        assert_eq!(TimeDelta::seconds(86_399).num_days(), 0);
+        assert_eq!(TimeDelta::seconds(86_401).num_days(), 1);
+        assert_eq!(TimeDelta::seconds(-86_399).num_days(), 0);
+        assert_eq!(TimeDelta::seconds(-86_401).num_days(), -1);
         assert_eq!(TimeDelta::days(i32::MAX as i64).num_days(), i32::MAX as i64);
         assert_eq!(TimeDelta::days(i32::MIN as i64).num_days(), i32::MIN as i64);
     }
@@ -705,7 +708,7 @@ mod tests {
         assert_eq!(TimeDelta::microseconds(42).to_string(), "PT0.000042S");
         assert_eq!(TimeDelta::nanoseconds(42).to_string(), "PT0.000000042S");
         assert_eq!((TimeDelta::days(7) + TimeDelta::milliseconds(6543)).to_string(), "P7DT6.543S");
-        assert_eq!(TimeDelta::seconds(-86401).to_string(), "-P1DT1S");
+        assert_eq!(TimeDelta::seconds(-86_401).to_string(), "-P1DT1S");
         assert_eq!(TimeDelta::nanoseconds(-1).to_string(), "-PT0.000000001S");
 
         // the format specifier should have no effect on `TimeDelta`
@@ -718,11 +721,14 @@ mod tests {
     #[test]
     fn test_to_std() {
         assert_eq!(TimeDelta::seconds(1).to_std(), Ok(StdDuration::new(1, 0)));
-        assert_eq!(TimeDelta::seconds(86401).to_std(), Ok(StdDuration::new(86401, 0)));
-        assert_eq!(TimeDelta::milliseconds(123).to_std(), Ok(StdDuration::new(0, 123000000)));
-        assert_eq!(TimeDelta::milliseconds(123765).to_std(), Ok(StdDuration::new(123, 765000000)));
+        assert_eq!(TimeDelta::seconds(86_401).to_std(), Ok(StdDuration::new(86_401, 0)));
+        assert_eq!(TimeDelta::milliseconds(123).to_std(), Ok(StdDuration::new(0, 123_000_000)));
+        assert_eq!(
+            TimeDelta::milliseconds(123_765).to_std(),
+            Ok(StdDuration::new(123, 765_000_000))
+        );
         assert_eq!(TimeDelta::nanoseconds(777).to_std(), Ok(StdDuration::new(0, 777)));
-        assert_eq!(MAX.to_std(), Ok(StdDuration::new(9223372036854775, 807000000)));
+        assert_eq!(MAX.to_std(), Ok(StdDuration::new(9_223_372_036_854_775, 807_000_000)));
         assert_eq!(TimeDelta::seconds(-1).to_std(), Err(OutOfRangeError(())));
         assert_eq!(TimeDelta::milliseconds(-1).to_std(), Err(OutOfRangeError(())));
     }
@@ -730,23 +736,26 @@ mod tests {
     #[test]
     fn test_from_std() {
         assert_eq!(Ok(TimeDelta::seconds(1)), TimeDelta::from_std(StdDuration::new(1, 0)));
-        assert_eq!(Ok(TimeDelta::seconds(86401)), TimeDelta::from_std(StdDuration::new(86401, 0)));
+        assert_eq!(Ok(TimeDelta::seconds(86401)), TimeDelta::from_std(StdDuration::new(86_401, 0)));
         assert_eq!(
             Ok(TimeDelta::milliseconds(123)),
-            TimeDelta::from_std(StdDuration::new(0, 123000000))
+            TimeDelta::from_std(StdDuration::new(0, 123_000_000))
         );
         assert_eq!(
-            Ok(TimeDelta::milliseconds(123765)),
-            TimeDelta::from_std(StdDuration::new(123, 765000000))
+            Ok(TimeDelta::milliseconds(123_765)),
+            TimeDelta::from_std(StdDuration::new(123, 765_000_000))
         );
         assert_eq!(Ok(TimeDelta::nanoseconds(777)), TimeDelta::from_std(StdDuration::new(0, 777)));
-        assert_eq!(Ok(MAX), TimeDelta::from_std(StdDuration::new(9223372036854775, 807000000)));
         assert_eq!(
-            TimeDelta::from_std(StdDuration::new(9223372036854776, 0)),
+            Ok(MAX),
+            TimeDelta::from_std(StdDuration::new(9_223_372_036_854_775, 807_000_000))
+        );
+        assert_eq!(
+            TimeDelta::from_std(StdDuration::new(9_223_372_036_854_776, 0)),
             Err(OutOfRangeError(()))
         );
         assert_eq!(
-            TimeDelta::from_std(StdDuration::new(9223372036854775, 807000001)),
+            TimeDelta::from_std(StdDuration::new(9_223_372_036_854_775, 807_000_001)),
             Err(OutOfRangeError(()))
         );
     }
diff --git a/src/traits.rs b/src/traits.rs
index 8a63300c..0018a7d9 100644
--- a/src/traits.rs
+++ b/src/traits.rs
@@ -316,6 +316,11 @@ pub trait Timelike: Sized {
     fn with_nanosecond(&self, nano: u32) -> Option<Self>;
 
     /// Returns the number of non-leap seconds past the last midnight.
+    ///
+    /// Every value in 00:00:00-23:59:59 maps to an integer in 0-86399.
+    ///
+    /// This method is not intended to provide the real number of seconds since midnight on a given
+    /// day. It does not take things like DST transitions into account.
     #[inline]
     fn num_seconds_from_midnight(&self) -> u32 {
         self.hour() * 3600 + self.minute() * 60 + self.second()