I've been following this - and have just been holding off contacting Apple, since it seems there's no fix yet from what some of you have gone through - But I've done a few tests tonight and can replicate the alert - here's what I found - maybe this can shed some light on the bug for some Apple tech:
Make event in Calendar - no alert
Open it on iPhone - shows alert at time of event
Edit it on iPhone - now in Calendar it shows Alert - Message 35791394 minutes after.
Edit on Calendar to set Alert to NONE,
iPhone still shows Alert at Time of Event
Edit iPhone to remove Alert to NONE
Calendar still shows NONE (as before)
Edit iPhone timing of event, still alert None
Calendar still shows NONE
---------NEXT-------
Make event in Calendar - no alert
iPhone shows Alert at Time of Event
Remove alert, set to None AND edit time of event on iPhone
Calendar still shows no alert.
-----So it seems to me that an event created first on Calendar with no alert - shows up on iPhone with Alert at Time of Event - then if you edit it on iPhone - it goes back to Calendar with the Alert -Message 35791394 minutes after.
But... if you FIRST remove the Alert on the iPhone before editing the time of event - it doesn't create an alert back in Calendar.