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:
where the list of additions corresponds to the values of F21, i.e 1,2,3,4.....