Try entering 10/4/1582 on the Julian Calendar: What is the JD? Now enter
10/15/1582 on the Gregorian Calendar: What is the JD? It should be one day
later, since Friday, 10/15/1582 (Gregorian) immediately followed Thursday,
10/4/1582 (Julian). Note also that if you switch between Julian and Gregorian,
there is a 10-day difference for a given date in 1582.
Warning: This calculator is not guaranteed for Gregorian dates
prior to 1582, nor for negative Julian Day Numbers.
Julian Day Number