zone.parse(timestamp); // 480

Parse an offset for a timestamp constructed from Date.UTC in that zone.

This is what Moment Timezone uses to parse input into a time zone. The process is conceptually similar to the following.

Assume we want to find the exact moment of March 19 2014 8:30 am in New York. Because the offset varies between -04:00 and -05:00 in New York, we don't know what the offset was on March 19th.

Instead, we create a timestamp in UTC and pass that to zone.parse, which will return the offset at that time.

var zone ='America/New_York');
zone.parse(Date.UTC(2012, 2, 19, 8, 30)); // 240

This is the code that handles the cases referenced in the Parsing Ambiguities section above.

var zone ='America/New_York');
zone.parse(Date.UTC(2012, 2, 11, 1, 59)); // 300
zone.parse(Date.UTC(2012, 2, 11, 2, 0)); // 240