Days & Time
How lunar days work and why some are skipped or doubled
Lunar Days (tshes pa)
Each Tibetan month has exactly 30 lunar days (ཚེས་པ, tshes pa), numbered 1-30. But these don't correspond one-to-one with solar days. Here's why:
A lunar day is defined as 1/30th of a synodic month (new moon to new moon)—approximately 23 hours and 37 minutes. Since this is shorter than a 24-hour solar day, the correspondence shifts over time.
When Days End
The key question for each date is: "When does this lunar day end?" The answer is calculated from the true weekday (gza' dag), which gives a precise moment in solar time.
If a lunar day ends after the following sunrise (traditionally ~5 AM local time), that lunar day "owns" that solar day. If it ends before sunrise, the previous lunar day owns it.
Duplicated Days (lhag)
Sometimes a lunar day is slow enough that it spans parts of two solar days. When this happens, both solar days receive the same lunar day number. This is called ལྷག (lhag), meaning "extra."
Example:
If the 15th lunar day ends late on Tuesday and again spans into Wednesday, both Tuesday and Wednesday might be labeled "15th day."
Omitted Days (chad)
Conversely, if a lunar day is fast, two consecutive lunar days might both end within the same solar day. When this happens, one lunar day number is skipped. This is called ཆད (chad), meaning "cut" or "omitted."
Example:
If the 8th lunar day ends early Wednesday and the 9th also ends on Wednesday, the calendar might jump from the 7th to the 9th, with no 8th day listed.
Tibetan Time Divisions
Traditional Tibetan astronomy uses a hierarchical time system:
| Tibetan | Sanskrit | Equivalent |
|---|---|---|
| chu tshod | nāḍī | 24 minutes |
| chu srang | pala | 24 seconds |
| dbugs | prāṇa | 4 seconds (one breath) |
One day contains 60 chu tshod, each containing 60 chu srang, each containing 6 dbugs. This sexagesimal system allows precise astronomical calculations.