Why Bhutanese Lunar calendar has Double and Omitted days?

If you are familiar with "Datho" (almanac) or following the Bhutanese Calendar you must have come across double days (2 days with single date) and omitted day (Omitted date, for example 17 today and 19 tomorrow with one day that 18 disappearing in between).

Well, first of all lunar month has only 30 days and in one year only 360 days whereas the solar calendar has 365 days with slight changes during the leap year.

Unlike lunar calendar, Solar calendar depends on the length of the solar day, measured from mean sunrise of one calendar day to mean sunrise of the next calendar day, which is approximately 24 hours.

Where as in the case of lunar days it depends on the relation of the Moon to the Sun. Moon goes faster in one part of its orbit and slower in another.

When Moon is in a particular lunar day at sunrise of one calendar day, and the Moon is moving so fast that it reaches the start of the next lunar day (not calendar day) BEFORE the next calendar day’s sunrise.

In this case short lunar day is not recorded whether on first or second calendar day, but is simply omitted from the calendar. This is represented by ཆད (CHEY).

On the other hand when the Moon is moving very slowly in its orbit, the same lunar day is noted at the sunrise of two calendar dates, thus effectively doubling that lunar date on the calendar. This is represented by ལྷག (Lhag).

Kindly note that omitted or doubled days are only an artifice of the solar calendar and no lunar days are either lost or doubled. They are all there.