From Bhagavata Purana 3.11.10:
यामाश्चत्वारश्चत्वारो मर्त्यानामहनी उभे । पक्ष: पञ्चदशाहानि शुक्ल:
कृष्णश्च मानद ॥ १० ॥
It is calculated that there are four praharas, which are also called
yāmas, in the day and four in the night of the human being. Similarly,
fifteen days and nights are a fortnight, and there are two fortnights,
white and black, in a month.