Merge and sum simultaneously

Hello,

I have two data sets that I would like to merge, but I would like a summation (from Dataset 2 below) to occur before the actual merger, and that value be added on (to Dataset 1). I have attached a fuller excel file that contains two example accounts, and the values that I'd like to be summed and merged.

Dataset 1 are monthly bills for a given account that have a start and end date (original data set has about 3000 accounts for 10 years)- each account is generally billed on different date intervals. Dataset 2 is a daily ET factor. The shared variable between the two data sets is microzone (of which there are about 50), and the date obviously plays a factor. For a given account and bill, I'd like to have the ET factor summed for those specific dates (or what falls between them), and merged to that account. Below is an example of what the data looks like- please let me know what you think is best, and thank you in advance.

Dataset 1 Dataset 2