Reference a value of previous year in a Script Logic

Hi,

I need to reference a value of previous year to that I have in a Script Logic.

I've tried it with "%YEAR% (-1)", but "%YEAR%" refers to the current year and doesn't refer to the year that I have in the Script Logic, that is, if I pass "2006.DEC " to Script Logic, the system returns me in "%YEAR%" the value "2010".

What syntax should I use?

Regards,

Miguel.