TI Echo Server Scheduling issue

Hello all,
I want to do a full backup at the start of each week (Sunday) and differential backups for the rest of the week (M-Sat).

Because the differential backup expects to know the name of the weekly base backup, my plan was to create one task for every week for full backups (week1.tib, week2.tib, etc.) and one task for every week for differential backups (pointing to the appropriate base backup).

My problem is that while I can do these on every first, second, third, and fourth week, there's nothing in the scheduler that allows me to select a fifth week backup. The only other option is the "last week" of the month (which most times will be the fourth week).

I should also mention that I'm trying to retain a month's worth of backups at any given time.

How would you do this? Thanks.