Indicator variables and calculating differences between numbers in a list

I decided to rewrite my question and change the title, to be more descriptive

Here is the problem:
I got three columns. The left one is the day number. The middle column is for indicator variables 1 or 0. This is suppose to be chosen by the solver function.

I want the third column to show the DIFFERENCE between the "chosen" days.
This can be better explained through an example:

07 | 0 |
10 | 1 | 10
14 | 0 |
17 | 1 | 7
14 | 0 |
17 | 0 |
21 | 1 | 4

How do I do this?

(extra information: the indicator values are chosen by solver based on several criteria not stated here, but this particular part of the sheet is suppose to refrain the chosen days to have differences between them larger or smaller than a given value.

Thank you so much in advance!

I assume your data to locate at A1:B7 , and the result at C1:C7
Plz do fllow steps

+ at cell C1, enter the formula

PHP Code: 

=IF(B1=1,B1,"") 


+ at cell C2, enter the formula

PHP Code: 

=IF(B2=1,A2-SUM(C1:C$1),"") 


+ at C3:C7 - Copy formula from C2

I hope that help you succesfully