Calendar event notification interval changing on it's own

[mccornwall]

Soemtimes I will set an event, sometimes an all day event in the calendar. On this event I put a 15 minute notification. I save it. Then, later, the notification for the event will go off, surprising me, since I know the event is for the next day...or 2 days away. When I open it and look, the notification has changed, all on it's own, to something like 24 hours. Or more.

WTF's up with that? How do I get it to actually keep what I enter for the notification interval.