This module adds a simple and custom field widget for field type Daterange when it uses a Datetime type, allowing users to define default start and end times. It also means you can leave the time input empty to fill later for the automatic time preconfigured in the widget.

UPDATED: please, use version 8.x-1.1 and ignore the initial one. That was just me, a newbie making the mistake to give a different drupal short module name from the one saved locally. My apologies.