monthly subtotals help

daredeviled

Hello! i'm having problem in my access program, here it is

main form:
ex;
name date 1500 -->fields
arwin 1/2/2009 200 --> records


subform: monthlysubtotal
ex;
month year 1500-->fields
jan 2009 10000--> records
feb 2009 0
i was able to compute for the subtotal but i cannot displayed it in monthly subtotals because it continously add each record. what i want is that after jan. 31 the subtotal will be displayed in a monthlysubtotal and when feb 1 comes it should restart to zero until i enter a new record in the main form.

Thanks and kindly help me.