yamas catvaras catvaro
martyanam ahani ubhe
suklah krsnas ca manada
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
It is calculated that there are four praharas, which are also called yamas, 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.