in the column next to the input (c1:c1000)? enter =c2-c1
and copy down this is the daily use
if C10 were the beginning of one month and C40 the beginning of the next month
=c40-c10 would be the use for that month
(Depending on how your month is defined It might be = C39-C9)
=(C40-C10)/30 would be the daily useage for month with 30 days