A lunar calendar (as you may well know) is different

There are three main calendar systems in use. We're familiar with the solar calendar.

A lunar calendar, such as the various Islamic calendars, has 12 lunar months in its year of approximately 342 days. Each month of 28 or 29 days follows the same cycle of the moon - some based on local astronomy and some based on Mecca's moon appearance. The "short" year explains why the month of Ramadan drifts around on our solar calendar. Beside the drift of seasons a given month doesn't have the same number of days year to year and it's nearly impossible to calulate the number of days between two dates + your birthday comes around a bit faster.

The lunar-solar calendar, such as the Hebrew calendar averts some of these shortcomings by adding a lunar month during various year to keep it closer to the solar calendar.

Then, of course is the Ethiopian calendar. I think it's still Julian and the year is off by 8 years IIRC due to a disagreement when Christ was born