Two ways of dividing time
Why do tithis and calendar dates not always align?
A calendar date—also called a civil date—runs from midnight to midnight. A tithi follows the changing Moon–Sun angle and can begin or end at any clock time, so their boundaries rarely line up.
Day 1 and Tṛtīyā. At this point, the calendar date is Day 1 and the lunar day is Tṛtīyā.
Reading the example
One tithi can cross two calendar dates.
The upper band divides time into Day 1 and Day 2 at midnight. The lower band divides the same stretch of time into Tṛtīyā, Caturthī, and Pañcamī. Their boundaries do not line up.
Caturthī begins during Day 1 and continues into Day 2. Day 1 therefore contains parts of Tṛtīyā and Caturthī, while Day 2 contains parts of Caturthī and Pañcamī. A tithi and a calendar date can overlap without sharing the same beginning and end.
This is the general principle behind apparently mismatched calendar labels. A rule for the timing of an observance that depends on a tithi at a particular time of day must check which tithi is present at that time. It cannot assume that one tithi fills one calendar date. Learn more about controlling times and special kinds of tithi.