Macro for Summing Date Column

I have a spreadsheets that are for billing customers. We bill them by the totals days they use our products (see Column G "Days" in the file).

Instead of manually adding each customer (I have 20-30 customers to do) I want to create a macro that will sum all the products.

Between each product are two blank rows. On the first blank row is the text "Subtotal". All other cells on that row are blank. I was thinking of using a do until loop inside a For next loop to add the days for each product group.

However, I haven't the foggiest idea how to put it together. Can anyone help me?

Thank you for your help.

Re: Macro for Summing Date Column

it is raw data. However, this sheet is going out to the customer as an invoice to be paid. So a pivot table won't work.

The formatting is not allowed to change.