Calendar Functions for dates starting in 1582

[Albert Chan]

(12-14-2021 11:20 PM)Matt Agajanian Wrote:  Although the date +/- days works with March 1, 1901-Feb 28, 2100, I need to calculate future/past dates starting with 1582. I’ve tried to simply add the amount of days plus/minus to the generated Julian/Gregorian Date Number, but that doesn’t work.

Greogrian calendar repeat itself, every 400 years = 365.2425 * 400 = 146,097 days
Within the 400 years, as long as we do not cross years divisible by 400, 1 century = 36524 days

We can thus "pull" any dates to within working range of Mar 1,1901 - Feb 28, 2100

   JDN(1/1/1983)
= JDN(1/1/1883) + 36524
= JDN(1/1/1783) + 36524*2
= JDN(1/1/1683) + 36524*3
= JDN(1/1/1583) + 146097