Bug 977050 - Automate changing end date/time in Calendar

This patch is just for ui review not code review currently.
only now that I realized that `getEl` and `_findElement` are different methods... now I feel bad for asking you to change the code.. I thought we could keep same markup and just add the selector to the `selectors` (like `_findElement` works).

but let's keep it like this for consistency with the other form fields..