[javascript] Get timezone from users browser using moment(timezone).js

All current answers provide the offset differece at current time, not at a given date.

moment(date).utcOffset() returns the time difference in minutes between browser time and UTC at the date passed as argument (or today, if no date passed).

Here's a function to parse correct offset at the picked date:

function getUtcOffset(date) {
  return moment(date)
    .subtract(
      moment(date).utcOffset(), 
      'minutes')
    .utc()
}