Wiktionary
tithi
n. The lunar date based on the Hindu calendar. There are 30 tithis in each lunar month. Tithis begin at varying times of a solar day and vary in duration from approximately 19 to 26 hours.
Wikipedia
Tithi
In Vedic timekeeping, a tithi (also spelled thithi) is a lunar day, or the time it takes for the longitudinal angle between the Moon and the Sun to increase by 12 °. Tithis begin at varying times of day and vary in duration from approximately 19 to approximately 26 hours.