Date/Time field with repeat allowing "last Monday" of month

My database tracks all servers and applications in our organization. The Server object has a date/time field to store each server's patching schedule. We need to post a calendar on our company intranet using this field to communicate downtime events due to the patching. The date repeat feature works beautifully for many of the records, i.e. First Monday of every 1 month. However, we have a good number of records where this field needs to be the Last Monday (or whatever weekday) of the month. This calendar is very important for our various department manager's for planning purposes. Thank you!