What is the right IF function to use to sum the last above cell has value and ignore zero?

Hi,
I need a hand to sort this formula the right way.
I am working with a call center workbook for daily agents performance and number of calls.
Call center system generates only cumulative data.
For example:-
1st day=50 , which means net calls = 50 as it is the 1st day and no other previous any calls.
2nd day=80 , which means net calls = 80-50= 30 net calls.
So I set a formula to calculate today cumulative calls volume minus yesterday cumulative calls volume as shown above.
My problem arise in case there is any day off for any agent.
This is current reads of the system and net calls calculated manually
success net
1st day 15 15 (1st day and no other previous any calls)
2nd day 28 13 (cumulative today success calls as read from the system =28) - (cumulative yesterday success calls as read from the system =15) = 13 (net calls)
3rd day 42 14 same
4th day 55 13 same
5th day 0 0 Agent is off
6th day 0 0 Agent is off
7th day 0 0 Agent is off
8th day 80 25 This is 80-55 which represent today calls - the last working day before the vacation of the agent which is 55, so 80-55=25 net calls.
This is my problem, what is the right formula function to use to calculate the net calls correctly and take agent vacation in consideration?
9th day 99 19 same
10th day 114 15 same
11th day 128 14 same
12th day 156 28 same

This is the excel sheet for this values to tell me the right thing.
NetCallsFormula.xls
https://www.dropbox.com/s/3ou1axh904...llsFormula.xls

So please what is the right formula to calculate the net calls and take this vacation into consideration?

Thanks