Calculating time that goes past Midnight

[jonvanwyk]

In the attachment, I show a timesheet with a drop down to select time of day. I cannot just use an equation that subtracts time, because night shift begins at 7pm or 11pm and works past midnight into the next day. Any idea how to make my drop downs in such a way that 7pm to 7am = 12 Hours? Thank you very much to anyone who can help!