The Julian calendar was proclaimed by Julius Cæsar in 46 B.C. and underwent several modifications before reaching its final form in 8 C.E. The Julian calendar differs from the Gregorian only in the determination of leap years, lacking the correction for years divisible by 100 and 400 in the Gregorian calendar. In the Julian calendar, any positive year is a leap year if divisible by 4. (Negative years are leap years if when divided by 4 a remainder of 3 results.) Days are considered to begin at midnight.

In the Julian calendar the average year has a length of 365.25 days. compared to the actual solar tropical year of 365.24219878 days. 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.

Julian Day

Astronomers, unlike historians, frequently need to do arithmetic with dates. 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 21:17 UTC. When is the next? 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. Julian days simply enumerate the days and fraction which have elapsed since the start of the Julian era, which is defined as beginning at noon on Monday, 1st January of year 4713 B.C.E. in the Julian calendar. 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!)--when you've become accustomed to rising after the "crack of noon" and doing most of your work when the Sun is down, you appreciate recording your results in a calendar where the date doesn't change in the middle of your workday. But even the Julian day convention bears witness to the eurocentrism of 19th century astronomy--noon at Greenwich is midnight on the other side of the world. 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. 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!

Modified Julian Day

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. A Modified Julian Day (MJD) is created by subtracting 2400000.5 from a Julian day number, and thus represents the number of days elapsed since midnight (00:00) Universal Time on November 17, 1858. Modified Julian Days are widely used to specify the epoch in tables of orbital elements of artificial Earth satellites. Since no such objects existed prior to October 4, 1957, all satellite-related MJDs are positive.

Adapted from Formilab's Calendar Converter.