Add comment to calculate UNIX_EPOCH_DAY

src/datetime/mod.rs

@@ -1903,6 +1903,14 @@ where
     }
 }
 
+/// Number of days between Januari 1, 1970 and December 31, 1 BCE which we define to be day 0.
+/// 4 full leap year cycles until December 31, 1600     4 * 146097 = 584388
+/// 1 day until January 1, 1601                                           1
+/// 369 years until Januari 1, 1970                      369 * 365 = 134685
+/// of which floor(369 / 4) are leap years          floor(369 / 4) =     92
+/// except for 1700, 1800 and 1900                                       -3 +
+///                                                                  --------
+///                                                                  719163
 const UNIX_EPOCH_DAY: i64 = 719_163;
 
 #[cfg(all(test, any(feature = "rustc-serialize", feature = "serde")))]