Convert General Number (##:##) to time format (mm:ss)

I am trying to set up a spreadsheet for our Physical Fitness Leaders and I am having issues with the calculation differences between run times. Trying to set it up so all they have to do is type in a general number as the time. I have the cell format set up as ##":"## so all they have to do is type 1030 and it comes out as 10:30. Then between the two run times I want to calculate the difference in seconds.


Don't know why this one is completely baffling me, but here I sit, asking for the assistance of the community.

Are the times entered h:m or m:s?

One way or the other:

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times are mm:ss

That would be the formula in line 2.

Beautiful, thank you.