I've been struggling with this myself because I have a database that was exporting an elapsed time as a date and time. I figured out the following single cell solution.

Assuming the date and time is in cell A1 use the following formula =MOD(A1,1). You can then format the results with whatever time format you want.

This works because the MOD function computes the remainder after dividing by the number to the right of the comma. In this case dividing by 1 leaves only the numbers to the right of the decimal.

In Lauren's example this leaves a decimal of 0.2565. When formatted with the appropriate time function you get the format Lauren is looking for.