Convert Month text to number

Hi

I have a simple spreadsheet that has a month column in text ie January, February.

How do i convert this to a number? So for January it would be 01, February 02 etc etc.

I've tried using AttributeValueMapper with the following source value;

@Value(Month)='January'

and a destination value of 01

but no luck.

Would be grateful if someone could suggest how i can do this

thanks

Simon Hume