--(MONTH(Cancellation!B16:B1013)=1)

Using the above with the SUMPRODUCT function would show results from January.
Now, I have a drop-down list that shows the months as text in Cell A1.
How can i adjust the above to use =A1 instead of =1?
Or what i need setup a seperate cell reference to use the number?
So if Cell A1=May then cell A2=5?
If so could you suggest a quick formula/function to convert May to 5 etc

Thanks,