I agree Timesum is your best bet for this as it is very flexible, you can either timesum over the whole time range in which case your formula would just be Timesum(Table A.Amount B) or you can specify the start and end dates after the Amount B. Here is a link to the Anapedis page which shows all the different options available to you:
Please review this information where it talks specifically about timesum(). Basically, you set the dates, start and end dates for the function to use. Also, please do not use timesum() on line item where Time is in the Applies To, instead think about using MovingSum().