Changed aroundtime to a time window (feature #1400)

@ProfYaffle & @adamsutton I rebased and cleaned up my version of the patch as discussed in the old pull request.
No value or 0 both mean midnight. It does not pose a problem for my changes, I can understand this could be an issue if other functionality tries to use this. I can refactor to return -1 if there is no correct value. I prefer to keep supressing 00:00 displays in the gui though.