Bhagavatapurana 3.11.10

3.11.10

yāmāś catvāraś catvāro martyānām ahanī ubhe /
pakṣaḥ pañca-daśāhāni śuklaḥ kṛṣṇaś ca mānada //

10. 11. The day and night of men consist of four Yamas each and a fortnight consists of fifteen such days and there are two such fortnights known as bright and dark. O Respected Vidura-these two together make a month which is a whole day of day and night for Pitris—Two such months make a season or Rutu and six months make an Ayana which is distinguished as the Southern and the Northern corresponding to the apparent course of the sun in the sky towards the north aud south of the equator.