You should just use the internal time functions
SECOND(). They're designed to get the minute/second component from a time value.
Actually, if you've got a problem with users entering the incorrect time format, the solution is not quite as simple as I wrote in the comment, but it's still fairly close:
This basically shifts each time component one unit to the right. The hours become minutes, the minutes become seconds.
You could use the
RIGHT() functions to do string manipulations and re-implement the time functions you need, but you'd essentially be fighting the software rather than working with it. And in the long run, you always get more done working with the software rather than against it.
If you still want to do it that way, it'd basically be something like:
=LEFT(A1, FIND(":", A1), -1)
=RIGHT(A1, LEN(A1) - FIND(":", A1))