Go to the Data Source tab in your workbook, ctrl-click all of the date columns then click the small triangle in the column header and select Pivot. This will make your data more of a flat file format and easier to use in Tableau.
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to andrew's point, your first step will be to pivot the data to get the month-year in a single column and the respective sales amounts in the other. your second step will be to split this month-year field. this will give you 2 fields now one for the month the other for the year. then you can create a date() field off of these split fields.
the data started out like this:
then we did the pivot in tableau to get this:
then we split pivot field names to get this:
then we built the above calculation to format this as a date field