An Indian almanac does not tell you the date. It tells you five things, and three of them are the Moon.

The panchanga — 'five limbs' — gives the tithi, the weekday, the nakshatra, the yoga and the karana. Tithi is the Moon's angle from the Sun, nakshatra is the Moon's position against the stars, and karana is half a tithi. A calendar built this way answers a different question from ours: not what number the day is, but what shape it is.