For my startup, I'm designing a recurring option that allows users to recur a payment method, one of which happens to be monthly (another is tri-monthly).
Weekly should be fairly simple, since it's always defined by a number of days, which are pretty unambiguous (timezones aside).
I see a few options here:
Use the "day of the month" (if 29+, treat cases for leap years passing through February, handle months with 30/31 days by adding an extra day on the beginning of the month?)
Add X calendar days to the sub_start_date ... except, now I've got the problem of determining if I should use 30 or 31?
I can think of a couple, less reasonable alternatives but I have a feeling it's one of these two, but I'm not sure what's standard.