Each rtu is a season of two months; India experiences six seasons, not four like Europe. The ayana is the apparent traversal of the Sun between northern and southern latitudes; uttaraayana (also called devaayana) is the traversal from south to north; dakshinaayana (also called pitraayana) is traversal from north to south.

Disparity between lunar (the months were lunar; 12 months came to 354 days, but the solar year was 11 days longer) and solar calendars were observed, and to resolve this disparity, which threw the calendar off, the concept of adhika maasa was introduced. Adhika means extra. Basically, it was a leap month, similar to a leap day.

A five-year cycle after which five solar years and 62 lunar months aligned again, was called a yuga (which means conjunction).

These demonstrate that the concept of time evolved even during the Vedic period.

When the yuga transformed into a tremendous period of 4,32,000 years and a chaturyuga 10 times as long is unclear; this is the period the puranas mention. The five-year yuga is not even referred to by astronomers of the Siddhanta or Classical period, except Brahmagupta, who rejects it, and the entire Vedanga Jyotisha, without explanation.

A fine historical and philosophical anomaly — the very Brahmagupta who is dismissed as orthodox for rejecting Aryabhata’s alleged heresies — is not himself considered a heretic for rejecting an entire Vedanga.

The invocatory sloka of Vedanga Jyotisha mentions all these divisions of the year, saluting Prajapati, the Creator, as one whose limbs are days, seasons, months et cetera.