Assign a value to a cell based on another in that row (please see explanation)

I have searched far and wide, and I see many things that come close, but nothing that solves my problem...

I work for a police department, and our city is broken up into reporting "districts." Every time a report is made, it is placed into an excel sheet and then sorted by date. Each row contains the report, and what district it is in. I would like to create a pivot table that can group multiple districts into groups "A", "B", and "C".

My problem is that the data is imported automatically from our mapping software, and my groups "A", "B", and "C" change weekly.

EXAMPLE:

This spreadsheet is huge. I'd like to be able to tell the sheet that for the range including dates 5/24 - 5/25, group all reports from district 05, and 13 into group "A". That way I can get a total of all reports from group "A" pretty easily in my pivot table.

Any help would be much appreciated!

Thanks!

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