[exim] Datestamp in filter log's filename?

I've been looking at the Exim filter spec (sec.3.15, "Logging
commands") and I'd like to include the year and week number in the log
filename: something like this

logfile $home/filter.${some.string.expansion}.log

which would then write the log output in "~/filter.2009w14.log" this
week, "~/filter.2009w15.log" next week, etc.

Is this possible?