Date-Time auto-fill

I have two date-time fields - Start and End. Start has the defaults set for current date/time. When I add a new record that’s what I get and want. End has no defaults. So when I edit a record and click on the date portion, the calendar pops up where the current date is highlighted (although not selected). When I click on the time portion it “defaults” to 12:00am. What I really want is to be able to set the current date time by clicking a button. I don’t want an edit rule that sets the datetime to current since the user may edit the record multiple times which have nothing to do with the end time. Is there a way to do this by clicking a button or link on the form or something?