Why a tithi is not a calendar date
A tithi, or lunar day, measures a 12-degree change in the Moon's lead over the Sun; it does not follow midnight-to-midnight dates.
STEP 1
Find the Moon–Sun gap
Use the Sun's and Moon's longitudes, their positions along the zodiac, for the same moment and zodiac setting. Subtract the Sun's value from the Moon's. If the result is negative, add 360°. A full 360° wraps back to 0°.
STEP 2
Count 12-degree intervals
Each tithi covers 12°, or 720 arcminutes. Arcminutes measure angles, not clock time. Thirty intervals fill one 360° cycle. Divide the gap by 12 and count the completed intervals; the current tithi is the next one.
STEP 3
Work an example
With the Moon at 68° and the Sun at 42°, the gap is 26°. Two 12° intervals are complete, with 2° left over, so this is the third tithi. It runs from exactly 24° to less than 36°. At 36°, the fourth begins.
STEP 4
Read the ending time
A tithi can change during a calendar date. When reading a panchang, check the listed tithi's ending time and the time zone used. Read any note about how dates are assigned, especially when an entry continues past midnight.