1) The most obvious problem is your assumption that adding the number 1230 to a date/time cell is the same as adding the
time 12:30 - What it actually does is to add 1230
days to the existing date/time.
2) Similarly the starting value in C21 should be shown as "
06:30" and should be formatted by Excel as a
time
3) Your formula could be expressed more concisely using Excel's CHOOSE function in this fashion:
=C21+ CHOOSE(F21,4:00:00,8:15:00....)
where the list of additions corresponds to the values of F21, i.e 1,2,3,4.....