Is there a reason why you're not using a parameter? And how are users selecting Month?
Depending on the method, you may be able to use a calculated field where [Month] is the result of User selection:
if [Month]=Jan then 1 else [Month]=Feb then 2... and so on.
Then use this calculated field in subsequent calculated fields.
I was looking at Parameters but I also need the Dimensions of the report to be filtered on the Month that they choose. I created the filter by stepping down a shipping date field to Month and then added it as a quick filter.
In essence I have the following. Pretend they selected September.
5-6 dimensions (This information filtered on September 2016)
YTD Sales (1/1/16 - 9/30/16)
PYTD Sales (1/1/15 - 9/30/15)
Current Month Sales (9/1/16 - 9/30/16)
Previous Month Sales (8/1/16 - 8/31/16)
I think you can use Month(Date) and functions like Datetrunc() and datepart() that solves your purpose.
Please find the complete list of tableau date fuctions here .Date Functions
Actually I was able to use a parameter and I created a calculated field which uses the parameter as a filter to filter the data for me.