[exim] user filter if $tod_log < "some time"

I have a user who turns on forwarding when he's off site, but then forgets to
remove it.

He is wanting to put in his .forward something to the effect of

if $NOW < '2016-10-20 17:00:00' then
deliver my_mobile@???
endif

deliver mylocaluser
seen
finish


I have found the variable $tod_log but I can't find out how to a string
comparison within a user filter. Is it possible?

Gary