Preschoolers understand that days are divided into mornings, afternoons, and nights and that years consist of four different seasons. When they enter kindergarten, children may know some days of the week and understand that time is measured by hours and minutes.
Shelov, S. P. (Editor-in-Chief). (2004). Caring for Your Baby and Young Child: Birth to Age 5. The American Association of Pediatrics. Revised edition. New York: Bantam Books.