Make an offset with a variable

Hi all,

In query designer, I want to make an offset with a variable, i explain :

Now, I modify my date manually. for example, i give in selection screen month and year, 05.2008 and i change this date in 04.2008 with a "-1" offset.

I want to give the possibility to make in the selection screen one numeric field which change automatically the offset :

date : 04.2008

offset : -2

=> date = 02.2008.

Can i do this we a variable?

Another way is to ask a second date. (date1, date2) but i prefer make this with an offset.

I can find anything to do this.

Can you help me?

Thanks,

Adrien