Do you let your network set the time or did you do it manually? Could be a hint that you're off by a minute or so (considering you've checked all the other reasonable things already).
Any other apps installed that may want to set alarms? Maybe a sideload? Calendar app you've tried and not uninstalled?
I fear as the number n of the repeated day approaches infinity, any day would be horrible to have to relive again. And again. And again.