How to schedule a Rules component at the end of the month?
Introducing last day of 23:59 works for me (last day of is the last day of the current month, and 23:59 just before the day ends). You can see the full reference of DateTime relative formats here: http://php.net/manual/en/datetime.formats.relative.php

In general, you can pass anything recognised by strtotime() as the scheduling date.