It's the timestamp from the SMSC (message centre) of lodgement,
giving the date, time, and timezone of lodgement.  So it's the
third of your six possibilities.  And of course, not all
receiving phones can (or do) correctly compensate for the
timezone component when displaying the message time.

Note that your fourth possibility is ruled out by the fact that
it's the SMSC of lodgement that delivers the message, even
across GSM networks.  If the recipient's phone is unavailable
on the first delivery attempt, the recipient's network should
notify the sender's network when that phone reregisters.

John