Hi Cristina,
How about creating Calculated Columns for Month and Year with condition for join as Table A.Month = Table B.Month and Table A.Year = Table B.Year?
You can use Year() and Month() function to get the Month and Year number, its available by default in IDT for some databases, check out if it works for the type of database you are using.
This can help to you create Calculated Columns using Date value:
Information design tool 4.0: Create a calculated column using a date value
Let us know if this helps and/or you have any questions/issues.
Update: If you've used Universe Design Tool (UDT) to create the Universe, then you'll have to create a Derived table.
Thanks,
Mahboob Mohammed