It appears that there’s a built-in “last day of month”.
Setting a schedule for: Monthly, Every 31 will do what you want.
Here’s the schedule, with next run of 5/12/19 at 4:30 PM (a few minutes after I created it)
And here it is after it fires.
Note that it set it to the last day of the following month. In this case June 30 (not June 31st).
And it maintained the time portion. So if you want it to run at 6:00 AM on the last day of the month:
- Schedule Type:
Monthly - Every:
31 - Set the next run to
5/31/2019 6:00 AM
It should run at 6:00 AM on 5/31, then set its Next Run to 6:00 AM on 6/30.