moment().utc();

Sets a flag on the original moment to use UTC to display a moment instead of the original moment's time.

var a = moment([2011, 0, 1, 8]);
a.hours(); // 8 PST
a.utc();
a.hours(); // 16 UTC

Utc can also be used to convert out of a fixed offset mode:

moment.parseZone('2016-05-03T22:15:01+02:00').utc().format(); //"2016-05-03T20:15:01Z"

See moment.utc() for more information on UTC mode.