Some comments:
I think the layout could be improved, e.g.:

Please note the spinbuttons for days.
There should be an easy way to enable/disable the addition/subtraction without manually setting the days and the time to zero (users even might forget to reset the fields and happen to add/subtract). The drop downlist could have an entry (e.g. "No +/-") which will disable the addition/subtraction. In this case, the fields "days" and "hours" would be greyed out.
XnView 1.82 alpha