Why a tithi is not a civil date
Lunar angular intervals cross midnight, sunrise and month boundaries in their own way.
A moving angular interval
A tithi spans 12 degrees of increasing Moon-Sun elongation. Since their relative motion is not uniform, a tithi does not last exactly 24 hours. It can begin in the middle of a civil day and end on the following one. Calendar interfaces must therefore show transitions rather than attach one permanent label to each date.
Repeated and skipped labels
When a tithi spans two local sunrises, a sunrise-based daily list can repeat its label. If a tithi begins and ends between two sunrises, the same list can omit that label. The tithi itself has not disappeared; the sampling convention explains the display. Festival rules need to handle these cases deliberately.
A month name also has a convention
Amanta and Purnimanta divide months at different lunar boundaries. Regional solar calendars follow other definitions. Language translation cannot implement a calendar system: the month rule, intercalation and observance convention must be real. Always retain the convention label when comparing a printed date with another calendar.
Original educational introduction · editorial review pending. Calculation limits are documented on the Methodology page.