[exim] System filter intelligence?

Hiya

I got the following in a system filter example at a client,

unseen deliver brentgclarklist@???

But I need to make it a little more intelligent.,

i.e. $year-$month-brentgclarklist@???

I tried googling, but its proving to be fruitless.

If anyone can assist, it would be appreciated.

Kind Regards
Brent Clark