acceptable timestamp of subscribed messages. For a new subscription,
the timestamp is initialised to the current time, so a new subscriber
to a group will not immediately receive any messages. (Subscribing to
a group is therefore more like joining a mailing list than joining a
Usenet group - you only receive messages written after you joined.)
Once the database fills up and starts expiring messages, the
timestamps of subscriptions are updated so that contacts need not send
messages that would expire immediately. This is done using the
*approximate* timestamp of the oldest message in the database, to
avoid revealing the presence or absence of any particular message.
