How to create custom date display format that displays event duration
Do the following:


Administration » Configuration » Regional and language » Date and Time
There you will see a tab for format.
Create a new format string "Y-M-d".
Make this as your default for all date fields and you won't see the seconds any more.
You don't need the date module for basic formatting.


UPDATE: From following comment,

If you want optional End time, use Timefield module


  Timefield is a Field API field for storing simple non-timezone time
  values, with an optional end time, utilizing a simple JQuery
  timepicker widget for input.