Format changes on fields don't affect their value. Perhaps you could post an example of the calculation field and then describe why it needs to be changed?
If you mean you want a TEXT result formatted as hhmm, try:
SerialIncrement ( "0000" ; 100 * Hour ( Timefield ) + Minute ( Timefield ) )
As already mentioned, any formatting applied to Timefield on a layout is irrelevant here. You can even place another instance of the field on the layout and format it as hhmm.