MATH or MAGIC!

The major components of the Pancanga/Almanac is vara (day of week), nakshatra (special constellation), yoga, tithi (lunar phase), karana (lunar half-phase).

In the Vedic system, the timekeeper is the Sun and the Moon, not the man-made clock.
The (solar) day is defined as the period between one sunrise till the next sunrise, not by human arbitrariness of 12 midnight changeover.