The other day a couple of pilots were treating 123.3 as their private chat line which I was forced to listen to so I could hear my friend's hourly position reports. One of the things one kept (repeatedly) talking about was having to descend to 12,500 to "reset" the 30 minute clock. This is entirely wrong: there is no "resetting" of the clock; it is a ONCE A FLIGHT exception, no doubt intended to give pilots w/o supplemental oxygen leeway in crossing high-altitude mountain passes. The governing FAR is 91.211:

§ 91.211 Supplemental oxygen.
(a)General. No person may operate a civil aircraft of U.S. registry -

(1) At cabin pressure altitudes above 12,500 feet (MSL) up to and including 14,000 feet (MSL) unless the required minimum flight crew is provided with and uses supplemental oxygen for that part of the flight at those altitudes that is of more than 30 minutes duration;

(2) At cabin pressure altitudes above 14,000 feet (MSL) unless the required minimum flight crew is provided with and uses supplemental oxygen during the entire flight time at those altitudes; and

(3) At cabin pressure altitudes above 15,000 feet (MSL) unless each occupant of the aircraft is provided with supplemental oxygen.