You can achieve that by typing something like [localtime://2012-10-16T11:52:14UTC] to the node, which will get formatted in the timezone of the reader, eg: Oct 16, 2012 at 11:52 UTC. Don't forget the UTC part or else the time is parsed as server time.
Here's a script to generate such a string.
perl -we 'use Date::Manip::Date; $d = Date::Manip::Date->new("now"); $
+d->convert("UTC"); print $d->printf("[localtime://%OUTC]\n");'
Btw, I don't usually give times like that in update notices, because I mark the update only up to a day of precision, or with no timestamp at all if I update soon after posting.