I find this obnoxious as well. Sure, one month has less than 29 days, but it seems pretty easy for data sense yo know that, for that one month, it will reset on the 1st of the next month rather than the 29th. It's a pretty stupid limitation.
Agreed, it's just dumb that they don't support dates after the 28th. I posted about it forever ago in Feedback myself as well.
That being said, it doesn't bother me nearly as much in W10M, since Edge doesn't have the same feature as IE11 did, where it could compress images to lessen data use when near your limit.