চান্দ্ৰ কেলেণ্ডাৰৰ লগত ঋতু পৰিৱৰ্তনৰ সামঞ্জস্য ৰাখিবলৈ কেতিয়াবা এটা অতিৰিক্ত মাহ যোগ দিয়া হয়। হিব্ৰু কেলেণ্ডাৰত এনে পদ্ধতিত ১৯-বৰ্ষীয় সময়চক্ৰ ব্যৱহাৰ কৰা হয়।
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===Lunar calendars===
{{Main|Lunar calendar}}
Not all calendars use the solar year as a unit. A [[lunar calendar]] is one in which days are numbered within each [[lunar phase]] cycle. Because the length of the lunar month is not an even fraction of the length of the [[tropical year]], a purely lunar calendar quickly drifts against the seasons, which don't vary much near the [[equator]]. It does, however, stay constant with respect to other phenomena, notably [[tide]]s. An example is the [[Islamic calendar]].
Alexander Marshack, in a controversial reading,<ref>James Elkins, ''[http://books.google.com/books?id=5Ku6YdWurMgC&pg=PA63&lpg=PA63 Our beautiful, dry, and distant texts]'' (1998) 63ff.</ref> believed that marks on a bone baton (c. 25,000 BC) represented a lunar calendar. Other marked bones may also represent lunar calendars. Similarly, Michael Rappenglueck believes that marks on a 15,000-year old cave painting represent a lunar calendar.<ref>[http://news.bbc.co.uk/2/hi/science/nature/975360.stm Oldest lunar calendar identified]</ref>