Comparing the Same Month Sales between Two Different Years

I am trying to create a function where the user can type in a single month, and 2 different years, and it will calculate/compare that single month from the 2 different years entered.

For Example: I type in January, and I type in 2014 and 2013. The excel formula will spit out whether January 2014 is over/under January 2013.

See attached for my sales report. You can see where I left some space for the requested function.

Please Please Please can you assist, as it is the last change I have to make to the sales report.

Thank you,

Yes the function is operating how i want it. I just need it to calculate for each individual tenant as opposed to the whole spreadsheet. The tenant names are all deleted but their unit numbers still are there. Is there a way you type in the tenant name and it will automatically reference it instead of the whole spreadsheet. So you would type in the month, the years, the tenant name and then it would spit out the comparison similar to what is id doing currently?

K64, I think this will work fine, even if it isnt referencing the data sheet. For future reference i will avoid merging cells.

Thank you so much, this has saved me a ton of time!