SB 3.11.10

yāmāś catvāraś catvāro
 martyānām ahanī ubhe
pakṣaḥ pañca-daśāhāni
 śuklaḥ kṛṣṇaś ca mānada
Synonyms: 
yāmāḥ — three hours; catvāraḥ — four; catvāraḥ — and four; martyānām — of the human beings; ahanī — duration of day; ubhe — both day and night; pakṣaḥ — fortnight; pañca-daśa — fifteen; ahāni — days; śuklaḥ — white; kṛṣṇaḥ — black; ca — also; mānada — measured.
Translation: 
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.