I will provide two personal examples. We had Preventive Maintenance in the Navy, and the cycles were simply stated as weekly, monthly, quarterly, or annually. So if I wanted to schedule a monthly, I would make the due date the end of the month. Granted we actually set up a daily schedule for what would be worked when.
When I was working at Department of Energy facilities, it was must more restrictive. If it was a monthly, that was to be based upon the last date it was completed. You could not do a PM on the First of April, and then the next on the 30th of May. There was also a complicating factor that some PM's had "Grace Periods" that allowed you to exceed the periodicity IF YOU NEEDED TO, but you absolutely could not exceed the grace period.