Skandapurana 16.105.35

16.105.35

triṃśatkalo muhūrtaḥ syāddinaṃ paṃcadaśaistu taiḥ..
dinamānā niśā jñeyā ahorātraṃ tayorbhavet ..35..

Thirty kalās make a muhūrta, and fifteen of them make a day;
the night is to be known as of the same measure as the day; the two make a day-and-night.