sum for past tweleve months

i feel a little retarded i can’t pull the available information together to
get the result i need. i have looked at
http://support.microsoft.com/kb/208714 and a number of other posts, but i
have not been able to get anything but the most recent sum for all Occur
Date. That equation is Expr1:
DSum("[HiredNum]","Firm_Count_Hired_Fired","[Hired.CurDate]=
[OccurDate]")=28 for every record.

i need to be able to get the gross hired/fired for an occur date but only
the last twelve months.
currently i have the first 4 columns below and used excel to show what the
answers would be:
OccurDate EmployeeCount HiredNum FirmNum Sum_ChangePast12Months
200710 1 1 0 1
200711 1 0 0 1
200712 1 0 0 1
200801 1 0 0 1
200802 1 0 0 1
200803 1 0 0 1
200804 1 0 0 1
200805 4 3 0 4
200806 4 0 0 4
200807 4 0 0 4
200808 5 1 0 5
200809 5 0 0 5
200810 6 1 0 5
200811 7 1 0 6
200812 10 3 0 9
200901 10 0 0 9
200902 11 1 0 10
200903 13 2 0 12
200904 14 1 0 13
200905 15 1 0 11
200906 16 1 0 12
200907 17 1 -1 12
200908 16 0 -2 9
200909 16 2 -1 10
200910 17 2 0 11
200911 20 3 0 13
200912 21 1 0 11
201001 24 3 0 14
201002 24 0 0 13

TIA