OK, from what you say you don't have a problem using NETWORKDAYS as long as it gets the right results? if start/end times can be any time then my earlier suggestion won't always work, try this version

=24*(NETWORKDAYS(AA2,AC2,AO$2:AO$21)+NETWORKDAYS(AC2,AC2,AO$2:AO$21)*(MOD(AC2,1)-1)-NETWORKDAYS(AA2,AA2,AO$2:AO$21)*MOD(AA2,1))

format result cell as number