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This date is defined in terms of a cycle of years, but has the additional advantage that all known historical astronomical observations bear positive Julian day numbers, and periods can be determined and events extrapolated by simple addition and subtraction.

Julian dates are a tad eccentric in starting at noon, but then so are astronomers (and systems programmers!

As in the Julian calendar, days are considered to begin at midnight.

But the Julian day notation is so deeply embedded in astronomy that it is unlikely to be displaced at any time in the foreseeable future.It is an ideal system for storing dates in computer programs, free of cultural bias and discontinuities at various dates, and can be readily transformed into other calendar systems, as the source code for this page illustrates.The calendar thus accumulates one day of error with respect to the solar year every 128 years.Being a purely solar calendar, no attempt is made to synchronise the start of months to the phases of the Moon.Use Julian days and fractions (stored in 64 bit or longer floating point numbers) in your programs, and be ready for Y10K, Y100K, and Y1MM!

While any event in recorded human history can be written as a positive Julian day number, when working with contemporary events all those digits can be cumbersome.In doing so, this implementation uses the convention that the year prior to year 1 is year 0.This differs from the Julian calendar in which there is no year 0—the year before year 1 in the Julian calendar is year −1. This page allows you to interconvert dates in a variety of calendars, both civil and computer-related.All calculations are done in Java Script executed in your own browser; complete source code is embedded in or linked to this page, and you're free to download these files to your own computer and use them even when not connected to the Internet.For example: a double star goes into eclipse every 1583.6 days and its last mid-eclipse was measured to be on October 17, 2003 at UTC. Well, you could get out your calendar and count days, but it's far easier to convert all the quantities in question to Julian day numbers and simply add or subtract.