column of cells representing minutes and secs is created how?

I have tried right clicking on a column the going to Format Cells -->Number-->Custom-->mm:ss .

This does not work though, because when I enter for example 10:20 wanting it to represent 10 minutes and 20 secs, when I move to the next cell the value changes to 20:00 in the cell and in the formula field it says 10:20:00 AM. This is not at all what I want. I am trying to create a column of entries that are mm:ss and then have an avg of these times at the bottom. I didn't think this would be very complicated to create but it it proving to be a major headache. Can anyone help me out.