Those people who know (the length of) the day- of-BRAHMA which ends in a thousand YUGAS (aeons) , and the night which (also) ends in a thousand YUGAS (aeons) , they know day-and-night.

sahasra = one thousand; yuga = millenniums; paryantaṃ = including; ahaḥ = day; yat = that which; brahmaṇaḥ = of Brahma; viduḥ = they know; rātriṃ = night; yuga = millenniums; sahasrāntāṃ = similarly, ending after one thousand; te = they; ahorātra = day and night; vidaḥ = who understand; janāḥ = people.;