Bug 977050 - Automate changing end date/time in Calendar

This patch is just for ui review not code review currently.
`_durationBetweenStartDateAndEndDate` is very verbose for not real gain (it is implied that *duration* is the difference between *start* and *end* dates..) - most of the line breaks on the code are caused because of this.

**protip:** if code can't fit code in a single line maybe you should store values into temporary variables or make names more concise. :P