I am trying to convert the name of a month to a number but there is a slight issue with using my normal method of TEXT("1/"&A1&"/2013","mmm")

I am trying to create a dynamic table which if you change 1 value for the month, it changes others

so my data looks like so,

Jan
Feb
Mar
Apr
May <---- this is the value which is chosen

the problem is when I select the month "January", it assigns the value of the above cell as 1/0/2013 which is not a date when in fact I want to set it as "December"

im not sure if I am making my problem clear but hopefully someone can understand where my problem lies.