entering a minus time value into a cell

Confusing title! I've nearly finished (with the help from forumites) my daughters timesheet.
The problem is cell N2. They use flexitime and some weeks they go into negative hours or minutes. This isn't a problem when doing calculations but when they first use this timesheet, they will need to transfer theiir records over from paper. The cell for the flexitime is N2 and this could be for example -3:05. but when I enter the minus sign before the time -3:05 it thinks it is a formula and won't accept it. Any ideas.
Thanks