From 6: 00 to 9: 00, epidermis is divided into basal layer, spinous layer, granular layer and stratum corneum. Cuticle cells are the main cell components of epidermis, accounting for more than 80% of the total number of epidermal cells. Corneal cells divide, differentiate and mature from basal cells, and finally fall off from the stratum corneum, which is a precise regulation process. Under normal circumstances, the newly-born stratum corneum cells gradually move upward in order, from the basal layer to the granular layer, it takes about 14 days, and then it takes 14 days to fall off on the stratum corneum surface, which takes 28 days. This is called skin passing time or replacement time. So the time for epidermis replacement is 28 days.