And to go into further detail with the altimeter/QNH setting, each body governing the airspace around the world (the FAA in the US, CAA in Canada, etc.) as well as other aviation bodies have designated a transition level between gauging height based on the local altimeter QNH setting, and height using the standard setting in thousands of feet (IIRC, only Russia uses meters for this; everywhere else uses feet). In the US, that transition level is 18000ft., based on the standard altimeter setting of 29.92 inches of mercury (QNH is used everywhere else outside north america, which standard is set to 1013. easiest way to convert between the two is 'altimeter setting / 2.954 = QNH'). So when descending out of FL180 (18000ft) to a lower altitude, the local altimeter setting for the destination airport, or closest weather reporting airport in the vicinity of the descent must be given. In your case of UAL430 flying from DEN to EWR, the controller may have said something like:
"United 430, descend and maintain one seven thousand, Newark altimeter two niner six four."
This is assuming the approach controller gave the pilots the Newark altimeter. That comes from the METAR report for that airport or reporting station.
BL.
