creating a formula that skips empty cells

im doing a meter reading on a daily basis but on weekends and sometimes during the week i dont do the reading. my problem is with the day following the missed read my calc is wrong. column c is the meter reading and column d is the usage. i get column d by subtracting the previous meter read from the current one (=IF(C9>0,C9-C8,0). my problem is after a non read day the usage ends up being the meter read. i need a formula that will search up column c until it finds a numerical figure to subract against the new read.
thanks
turbo90talon