How to calculate no.of joining person???

Hi Expert,

I have Bex query requirement where in i have enter 0calmonth(interval-selection screen) and display the no. of joining person for last 12 month and include the current month. ie example

User enter calmonth- april2013 to nov2013 so result wiil be come like

Company code     calday                  No.of joining person

1000                   April13                       500                          ( eg.May2012 to april2013)

2000                  May13                         600                         (eg. June 2012 to May 2013)

300                   june 13                         700                     (eg.July 2012 to june 2013)

:                        :                                  :                               

:                        :

:                        :

800                   November 13             1000                         (   December 12 to November 13)

so how to calculate no of joining person..?????

Pls Advice me ASAP

Regards

Waseem