Using year (yyyy) from date field

Using year (yyyy) from date field

Hi everyone, I want to know if possible from a date field to take only the yyyy portion and store that into another filed, let say a number field.
 
I have a form with  Invoice Date (i.e. 30 April 2011) and want to store 2011 into other field, can someone tell me how to do that?
 
Thanks a lot.