Limit Input On Timecards

Currently our employees enter their time onto their timecards by the hour and quarter hour. However, on occasion, we have employees who use .15 instead of .25 or .05 instead of .50. Some of these errors are new employees who need further training and some are just mis-keyed. It would be wonderful if there was a way to limit the input on timecards to only accept quarter hours (.00, .25, .50, .75). Thank you!