Can you show me how to remove Sections for Previous Function??

I have a Report of Daily Water Consumption that:

Daily Consumption = Water Meter Indicator (Today) - Meter Indicator (Previous day)

Ex:

Month Date Meter Today Meter Previous day Daily Consumption

11 28/11 5.63 0 5.63

11 29/11 6.10 5.63 0.47

11 30/11 6.54 6.10 0.44

12 1/12 6.70 6.54 0.16

12 2/12 6.80 6.70 0.10

12 3/12 7.00 6.80 0.20

But when I set Month as a Section or I just show report for December only, the value is wrong for the 1st day of the month as below:

Section 1: 11

Date Meter Today Meter Previous day Daily Consumption

28/11 5.63 0 5.63

29/11 6.10 5.63 0.47

30/11 6.54 6.10 0.44

Section 2: 12

Date Meter Today Meter Previous day Daily Consumption

1/12 6.70 0 6.70

2/12 6.80 6.70 0.10

3/12 7.00 6.80 0.20

How can I solve this situation? Which function should I use to remove Sections for this previous value??

Thank you in advance for your help!

Van.