=IF(OR(D41="",F41=""),0,(NETWORKDAYS(D41,F41)-1)*("15:00"-"06:30")+MOD(F41,1)-MOD(D41,1))

Im trying to calculate the non-working time accumulated by a process
outside the normal shift of M-F 0630-1500 timeframe. But if all the work is
done on a non-working day (Saturday or Sunday) I get the negative dates or
time symbol ########, or in some cases hours after or before the time above
for M-F. Ive checked the dates and theyre correct.

Can someone explain possibly other reasons why this may be occurring?

Mike