Julian Day Converter

In short A Julian Day is a continuous count of days since noon Universal Time on 1 January 4713 BC in the Julian calendar. It has no months, no leap years and no reform gap, which is exactly why astronomy uses it: the interval between any two events is a subtraction, and no calendar rule can interfere. J2000.0, the epoch nearly every modern formula counts from, is JD 2451545.0 — noon on 1 January 2000. The Modified Julian Day is JD minus 2,400,000.5, which both shortens the number and moves the day boundary to midnight, and it is what most modern software uses. The half-day offset trips people up constantly and it is not a bug in anyone code: a Julian Day was defined to start at NOON, because it was built for astronomers who work through the night and would rather the date did not change in the middle of an observing session. So a whole number is midday and a number ending in .5 is midnight. There is one place a converter has to be careful. When the Gregorian calendar was adopted, Thursday 4 October 1582 was followed directly by Friday 15 October 1582 — ten days were removed to correct the drift the Julian calendar had accumulated. Those ten dates never existed, and a converter that silently returns a number for 10 October 1582 is describing a day nobody lived through. This one refuses them. Julian Day numbering itself runs straight across the reform without a gap, and so does the weekday cycle: 4 October was a Thursday and 15 October was a Friday.