Better date formatting for relative dates in task element

When configuring relative dates in the course element task or group task one case select dates depending on the course date.

Course dates do always start at 0:00. When configuring +5 days the meaning is +5x24h. Example: Course starts a 1.1.2016, plus 5 days means the closing date for the submission is 6.1.2016 at 0:00.

This is missleading because users don't read the 0:00. The think the full 06.01.2016 would be included.

The solution is to check for 0:00 times in dates and in such cases to subtract one day and to format the date the following way:

One way of doing this could be to replace "HH" with "kk" and "KK" with "hh" in the Formatter. (did not test if this does the trick already)