Year-Month Reference Field

I have a table called Orders.  Fields include Creation Date (YYYY-MM-DD), Total Revenue, and Total Refunded for each customer's order.  I need to create a new table to sum the Total Revenue and Total Refunded by Year-Month Combo.  I'm doing this so I can figure out the Average Refunds per Month (which I apparently can't do using a Summary Table on the Orders table).

I'm stuck trying to figure out how to create a field on the Orders table that is just the Year-Month Combo (date field?  text field?) that I can then use as the reference field on the parent Year-Month Table (can I use a formula field as a reference field?).  

Thanks for any clarity you can provide.

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Kevin BP
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