Excel 2010: Formatting minutes and seconds

I have a sheet where staff enter the time it takes to do a task. I'm using a custom format of [mm]:ss. However, I enter (for instance) 5 into the cell, the value becomes 7200:00. I finally figured out that this is because that is the number of minutes in 5 days. However, I don't want staff to have to enter 00:05:00 to indicate they worked for 5 minutes. I would rather them be able to enter something like 5:30 (or something along those lines) to indicate this took 5 minutes and 30 seconds. Would someone mind giving me some help? Thanks!