how can i calculate a quaterly average on a prorata?

[darkphoenix]

Dear All,

i would like to calculate a average based on a sum from a 3 months period. the tricky part is that my first month was started on the 25!!!

exple
A1 = 01/08/2012 B1= 1000 C1=pro rated araverage sum of (B1:B3) ?
A2= 02/01/2012 B2= 1200
A3= 03/01/2012 B3= 1500

also
how can automatically i change the number from

A1= 01/20/2012
TO
A2= 02/01/2012
A3= 03/01/2012
A4=04/01/2012
A5= 05/01/2012
A6= 06/01/2012
A7= 07/01/2012
A8= 08/01/2012
A9= 09/01/2012
A10= 10/01/2012
A11= 11/01/2012
A12= 12/01/2012

...
on a twelve month basis??

IS it possible?? what would be the formula? please help me!
i greatly appreciate your help and support!!
i love u guys!!!