Gaius Julius Caesar
The Julian calendar has a regular year of 365 days divided into 12 months, as listed in Table of months.
A leap day is added to February every four years. 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 calendar 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 inserts leap days according to a different
rule. Consequently, the Julian calendar is currently 13 days behind the Gregorian calendar; for instance, 1 January in
the Julian calendar is 14 January in the Gregorian.