View Source DateTimeParser.Parser.Serial (DateTimeParser v1.2.0)

Parses a spreadsheet Serial timestamp. This is gated by the number of present digits. It must contain 1 through 5 digits that represent days, with an optional precision of up to 10 digits that represents time. Negative serial timestamps are supported.

Microsoft Excel has, since its earliest versions, incorrectly considered 1900 to be a leap year, and therefore that February 29, 1900 comes between February 28 and March 1 of that year. The bug originated from Lotus 1-2-3 and was purposely implemented in Excel for the purpose of backward compatibility. Microsoft has written an article about this bug, explaining the reasons for treating 1900 as a leap year. This bug has been promoted into a requirement in the Ecma Office Open XML (OOXML) specification.

Microsoft Excel on Macintosh defaults to using the 1904 date system. By default, this parser will assume the 1900. If you want to opt-into the 1904 date system, see the t:use_1904_date_system option.

See more at https://en.wikipedia.org/wiki/Leap_year_bug