How to sum data for the last twelve months on a sheet that is updated frequently.

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I have a sheet that collects monthly sales and I want to be able to sum the previous 12 months for a specific set of markets. For example, if I wanted to sum the last 12 months for Aiken and Bamberg how would I write a formula that would update as changes are made to the sheet. Each month new data is entered and I want a formula that I can use on a collector sheet to sum only the most recent 12 months as new data is entered. This formula has to be able to take only the previous 12 months as the sheet changes because the sheet is not static. Thanks in advance !


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