serial_from_ymd worked out the real calendar date and then moved serials
up to 60 down by one. February 1900 has no 29th in the real calendar, so
DATE(1900,2,29) and DATEVALUE("2/29/1900") rolled over to 1 March (61),
and DATE(1900,3,0) gave 28 February (59). Excel's 1900 system keeps the
Lotus leap day: serial 60 is 29 February 1900, and DAY(60) already
said so.
The serial of the first of the month is now taken in Excel's calendar
(one lower up to February 1900), and the day, with any overflow, counts
on from there. So day and month overflow pass through serial 60 both
ways: DATE(1900,1,60), DATE(1900,4,-31) and DATE(1901,-10,29) are 60 too.
Month and day arithmetic is checked, so hostile values return None.
days_in_month_in gives February 1900 29 days in the 1900 system; EDATE
and EOMONTH use it, so EOMONTH(60,0) is 60 rather than the day before,
and the text parser uses it in place of its own special case. The 1904
system is unchanged (1900 is before its epoch).
Tests: serials around the leap day (overflow both ways, every serial
0-366 round-trips through its year, month and day), hostile months and
days, parsing 2/29/1900 and 1900-02-29, DATE/DATEVALUE/EDATE/EOMONTH
in the 1900 system and the 1904 system. The DATE, DATEVALUE and serial
cases fail before the fix.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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