when "collecting an end date", make start date equal end date but allow different start and end TIMES

I have a pretty common scenario where a user is entering the date of an event. All events occur on the same day (no single event spans 2 days), however, some events repeat for multiple days.

I do not want the user to enter a start date and end date that are 3 days apart, rather, to enter one date that repeats for 3 days.

For example, in node/add form, the user is presented with:

Date Time

to

Date Time

However, I would like the user to see:

Date Start Time to End Time

Effectively, I want the form to assume the start date and end date are the same. Only the TIMES will change. How could this be achieved? hook_form_alter()?