Halakhah zu Wajikra 22:6

I investigated further and found that with regard to one who has a discharge in the night or in the day, or one who touches any contamination which renders him unclean, Scripture says “he is unclean until dusk” (Leviticus 22:6), which must be the end of the day. For if the day began with dawn, then one who has a discharge at night should become clean at the end of the “day,” namely, at dawn.