Problem with Scheduled Maintenance Options

I am experiencing an issue with establishing Scheduled Maintenances for my client.

They perform "filter changes" every six weeks, but because zFSM has built in intervals it seems impossible to schedule.

Options are currently:
Every 1 Week

Every 2 Weeks

Every 3 Weeks

Every 4 Weeks

Every 5 Weeks

Days are the same way, so we can't even use 42 days.

I am not sure why zFSM doesn't just have user select Days, Month, Year and then prompt for a number, but this pre-defined method does not work.

And as a side note: why are scheduled maintenances hidden away in settings when everything else is under WO management. Seems like a strange place to access that.