Thanks pbaldy, that works, if I only had those two fields, but my actual calculation on my textbox is:
=([Todate]-[Fromdate]+1)*[Rental Fees]*[Quanity]
I tried this expression: =sum(([Todate]-[Fromdate]+1)*[Rental Fees]*[Quanity])
The problem is with the date calculation, as I tried that alone being:
=sum([Todate]-[Fromdate]) still get error return