The Julian date code assigns a three-digit number to each day of the year, from 001 (January 1st) to 365 (December 31st).

For the day-number calendar used for astronomical and historical calculations, see Julian day. It took effect in 45 BC (AUC 709), shortly after the Roman conquest of Egypt.

It was the predominant calendar in the Roman world, most of Europe, and in European settlements in the Americas and elsewhere, until it was refined and gradually replaced by the Gregorian calendar, promulgated in 1582 by Pope Gregory XIII.

The Julian calendar gains against the mean tropical year at the rate of one day in 128 years.

For the Gregorian the figure is one day in 3,226 years.

The difference in the average length of the year between Julian (365.25 days) and Gregorian (365.2425 days) is 0.002%.

The Julian calendar has a regular year of 365 days divided into 12 months, as listed in Table of months. The Julian year is, therefore, on average 365.25 days long.

It was intended to approximate the tropical (solar) year.

Although Greek astronomers had known, at least since Hipparchus, a century before the Julian reform, that the tropical year was a few minutes shorter than 365.25 days, the calendar did not compensate for this difference.

As a result, the year gained about three days every four centuries compared to observed equinox times and the seasons.

This discrepancy was corrected by the Gregorian reform of 1582.

The Gregorian calendar has the same months and month lengths as the Julian calendar, but, in the Gregorian calendar, years evenly divisible by 100 are not leap years, except that years evenly divisible by 400 remain leap years.

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