Hook when event date has changed?

Hey, I’d like to send E-Mail nnotifications when an event date has changed. But there seems to be no default hook or filter? save_pre hook is too late, cause the event-date has been saved already. Can somebody point me to the right direction? How to recognize when a date (-time too) has changed?

I dont want to implement some hidden fields (in all these different views) in custom view templates. This seems to be weired…