I've come up with the following, which will work. Can anyone suggest a *nicer* way?
Gary
# Exim Filter <== Do not edit this line
#
# Created 2016-10-19
deliver gary1
if $tod_zulu matches "^(.{12})" then
if $1 is below 201610201032 then
unseen deliver mandy
endif
endif
seen
finish
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