Flowfield Issue

I wondered if anyone could help. I have a field flowfield in table A that sums up the quantity from a decimal field in table B. However i only want to include it in the flowfield records that have a date greater or equal in table A than a date held in table B.

Example 2 records in table B

010108 - 10
010208 - 15

Records in table A

010108 - Flowfield should read 25
011508 - Flowfield should read 15 (ignoring the first record as the date is greater).

Can anyone help?
Regards,