Object dependency to read substring

Hi.

Is it possible to evaluate a substring in an object dependency? I want to write a simple OD that defaults a char field to a month code (the 2 digit month) based on another field which stores a date.

Something similar to this:

$set_default ($self,Z_PUB_MNTH,'01')

    If Z_ON_SALE_DATE+4(2) = '01'                   'date stored as string yyyyddmm

I'm getting these errors:

E28031 Comparison operator required

E28024 Syntax error in simple action/procedure

Any help would be much appreciated. Many thanks.