Cyclic periods
Examples of cyclic periods are a day, a week, and a fortnight, with fixed intervals of 1 day, 7 days, and 14 days, respectively. An academic semester cannot be described as a cyclic period, because spring, summer, and fall semesters have different lengths.
This example shows a lunar calendar month, assumed to be 28 days:
Cyclic period: work days and free days
PERIOD NAME = MOON
PERIOD TYPE = A
INTERVAL ORIGIN = 7 February 2003 (date of a new moon)
INTERVAL = 28 days
This example shows a period that is useful if you need to take a backup every 4 work days:
Cyclic period: work days only
PERIOD NAME = BACKUP
PERIOD TYPE = W
INTERVAL ORIGIN = 2 January 2003
INTERVAL = 4 work days