As I recall, the Julian Calendar has leap years, in fact, I think it is a matter of seconds in the leap year that distinguish it from the Gregorian Calendar. Informed comments welcome.
The Julian Calendar considers every year evenly divisible by 4 to be a leap year.
The Gregorian Calendar does the same except for those years divisible by 100, which it treats differently. On this calendar, only those century years divisible by 400 count as leap years. Julian Gregorian
1800 - Yes No
1900 - Yes No
2000 - Yes Yes
2100 - Yes No
As a result, the Julian and Gregorian calendars grow three days farther apart every 400 years. You may never notice a change in your lifetime, but if you look back over the centuries, you'll see a big change.
The Revised Julian
Calendar, the calendar currently in use in the New Calendar churches, is even more accurate than the Gregorian Calendar, however, by an order of about 3-4 days per millennium.