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)