The Gregorian calendar is based on a solar year–the amount of time it takes the earth to revolve around the sun– roughly 365 days. Tibetan calendars use lunar months, which are based on the time it takes the moon to revolve around the earth–roughly 29.5 days. A true lunar year–composed of 12 cycles of 29.5 days––is five days less than a notional lunar year of 360 days (12 x 30 days).

This loss results in an adjustment that takes into account the moon’s progression in relation to the sun. If the moon does not advance by 12 degrees on a given solar day, the Tibetan calendar marks the same day twice, and if the moon advances through two 12-degree quadrants on a solar day, the Tibetan calendar skips a day. As a result, in a typical Tibetan year, there will be roughly 7 duplicated dates and roughly 12 omitted ones. This net loss of 5 days is the difference between a **notional lunar calendar year** (360 days or 12 x 30 days) and the **true lunar year** (354 days or 12 x 29.5 days).

The **true lunar year** (354 days) is still 10.8 days shorter than a **solar year** (365). This net loss of 10 or 11 days every year is made up by inserting an extra or intercalary Tibetan month every 2 or 3 years as needed. As a result, the Tibetan dates will shift backwards in relation to the Gregorian year for a few years, and then suddenly jump forward again when the intercalary month is added. Thus, while our navigational roses show the 1st Tibetan month to start on Feb 21st, the actual time that the Tibetan new year falls can range between early February and mid March. The Tibetan New Year fell on Feb 25th in 2009 and Feb 14 in 2010.