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Time in UNIX system calls counts from the epoch, 1970-01-01. The timespec struct used in various system calls represents this as a number of seconds and a number of nanoseconds. Nanoseconds are required to be between 0 and 999_999_999, because the portion outside that range should be represented in the seconds field; if nanoseconds were larger than 999_999_999, the seconds field should go up instead. Suppose you ask for the time 1969-12-31, what time is that? On UNIX systems that support times before the epoch, that's seconds=-86400, one day before the epoch. But now, suppose you ask for the time 1969-12-31 23:59:00.1. In other words, a tenth of a second after one minute before the epoch. On most UNIX systems, that's represented as seconds=-60, nanoseconds=100_000_000. The macOS bug is that it returns seconds=-59, nanoseconds=-900_000_000. While that's in some sense an accurate description of the time (59.9 seconds before the epoch), that violates the invariant of the timespec data structure: nanoseconds must be between 0 and 999999999. This causes this assertion in the Rust standard library. So, on macOS, if we get a Timespec value with seconds less than or equal to zero, and nanoseconds between -999_999_999 and -1 (inclusive), we can add 1_000_000_000 to the nanoseconds and subtract 1 from the seconds, and then convert. The resulting timespec value is still accepted by macOS, and when fed back into the OS, produces the same results. (If you set a file's mtime with that timestamp, then read it back, you get back the one with negative nanoseconds again.) Co-authored-by: Josh Triplett <josh@joshtriplett.org>