Microscopic examination of a suitable, stained specimen from the endometrium to establish the number of days to the next menstrual period. The dating is based on an ideal 28-day cycle. Thus, day 8 indicates menstruation is 20 days away, and day 23 that it is 5 days away. This system was devised by the late Dr. John Rock, a physician at Harvard Medical School, to enable gynecologists to visualize endometria being discussed without having to provide detailed descriptions of the material studied.