[AccessD] Quarter hour increments

Hi all. I've got a database that lets people enter the amount of time they 
spent on a particular project. To make it easier on those of us who have to 
work with the data after they've entered it, we'd like to restrict them to 
quarter hour increments. So, they could enter 1.25 or .5 or 3.75 or 5 but 
never 1.08 or 2.66. Can I restrict their entries in that way?

Thanks!
Gina