Julian calendar

noun
1.
the calendar established by Julius Caesar in 46 b.c., fixing the length of the year at 365 days and at 366 days every fourth year. There are 12 months of 30 or 31 days, except for February (which has 28 days with the exception of every fourth year, or leap year, when it has 29 days).
Compare Gregorian calendar.
British Dictionary definitions for Julian calendar

Julian calendar

noun
1.
the calendar introduced by Julius Caesar in 46 bc, identical to the present calendar in all but two aspects: the beginning of the year was not fixed on Jan 1 and leap years occurred every fourth year and in every centenary year Compare Gregorian calendar