At the very least you want to follow Paul's suggestion of removing the server from the farm first. But doing this also removes any configuration, installed solutions, etc. At this point I would question the value of the effort. It would be best to do a new install to ensure you have all necessary patches/updates installed.

The all day event is a boolean type so you could do a logic check on whether that field equals yes. If so, update the appropriate timeoff value for a full day's time. If it says no, subtract the alternate selected amount.