For astronomical code (and any sane date code) you use Julian Day Number which is the number of days since a defined day 0. The one most seen would be Microso

For astronomical code (and any sane date code) you use Julian Day Number which is the number of days since a defined day 0. The one most seen would be Microsoft DateTime first in Excel where day 1 is 31st Dec 1899 in USA/Georgian Calendar - but only useful from March 1st 1900 Wikipedia gives the Astronomical Day number as The Julian day number (JDN) has the same epoch as the Julian period, but counts the number of days since the epoch rather than the number of years since then. Specifically, Julian day number 0 is assigned to the day starting at noon Universal Time on Monday, January 1, 4713 BC, proleptic Julian calendar (November 24, 4714 BC, in the proleptic Gregorian calendar).[4][5][6][a] For example, the Julian day number for the day starting at 12:00 UT (noon) on January 1, 2000, was 2451545.[7] There is the Modified Julian Dates several of which including the Microsoft one are listed in Variants here https://en.wikipedia.org/wiki/Julian_day https://en.wikipedia.org/wiki/Julian_day