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