Add time spent on a past date

Discussion

From https://gitlab.com/gitlab-org/gitlab-ee/issues/1265#note_18833868

If I forgot to log my time for last week, the UI lets me back-fill WHEN the time was spent because I can enter the date that the time was spent. In the current Gitlab EE implementation it seems that you can record that time was spent, but not on what days. My boss would not like losing the information that would be lost moving from our Redmine solution to the current Gitlab 8.14 EE one.

Proposal

Ability to indicate on which date the time was spent.