For a classification of calendars see Types of Calendars .
Most years have twelve months, but some years have thirteen.
All odd-numbered months have 29 days and all even numbered months have 30 days, except that the 13th month in a long year has either 30 or 31 days. This completes the definition of the Meyer-Palmen Solilunar Calendar. This calendar was invented by Peter Meyer in March 1999 starting from a general form of the above rule for long years etc. Some of these are discussed further in subsequent sections. In one era there are exactly 6840 years, of which 2519 are long years.