I need to format a range of cells as mm:ss. I've tried using the custom
format setting for cells and the values I've added are rounded. If I add a
new column and format it first, then add the numbers, they are interpreted
weirdly. 11:36 becomes 38:24.0. In either case, when I put the cursor on the
cell itself, the number shows as a either a clock time (11:36:00 am or
1/11/1900 8:38L24 AM),

I've tried several of the different tips suggested in this discussion area
(for example copy and paste special/ divide) and none have helped. What else
can I try?

This is for my son's science project and not getting the data displyaing
correctly impacts his graphs.