Automatically number rows in Excel according the dates in column B

Automatically adding serial numbers in Excel is easy, but I would like to know if there is an easier way to add serial numbers only columns near new dates. I simple works, want to add serial numbers to each days, without any counting duplicates. Each date should be counted only once.

I am only able to achieve the following.

But I would like to get the following results, just by dragging from the top column.