Make End Dates optional and add assistance for entering them

(this has some overlap with /node/141598 but it's not the same)

I like the way the All day checkbox removes the controls for the hours, and I'd like to see something similar for the End date block. Most of my events don't have a published end time. I can get Events to hide the End date by setting it the same as the Start date, but having to do this over and over quickly becomes tedious. Also, virtually all of my events end on the same day, and having to select the same day twice (for start and end) is tedious, too.

Here's an idea for a user interface:

1. Add a checkbox to the Start date group:
   [ ] specify end date
   leave it unchecked and hide the End date group.
2. If the user doesn't check the box, then store an instantaneous (or all-day) event.
3. If the user checks the box, then make the End date group visible and set the controls to the same values as the Start date group; these are much better starting values than the current date/time, as it is now.

If I understand the OP in /node/141598 correctly, then he would want the All day checkbox checked and hidden, and the proposed specify end date checkbox unchecked and hidden.