getUtcOffset.js

// tzName is one of the tz database names at https://en.wikipedia.org/wiki/List_of_tz_database_time_zones
// Example: 'Pacific/Honolulu' (UTC-10)
function getUTCOffset(tzName, date) {
  date = new Date(date || Date.now());
  date.setMilliseconds(0);

  // 1) Get the local UTC offset
  // ---------------------------
  // Convert date UTC offset to true hours offset.
  // Date.prototype.getTimezoneOffset() confusingly returns positive
  // values for earlier time offsets and negative for later.
  const localOffsetHrs = (date.getTimezoneOffset() / 60) * -1;

  // 2) Get a parseable date string for the provided date's time in the desired timezone
  // -----------------------
  // new Date().toLocaleString('en-US', { timeZone: 'Pacific/Honolulu' } )
  // -> "5/3/2021, 11:04:57 AM"
  const thereLocaleStr = date.toLocaleString("en-US", {
    timeZone: tzName
  });

  // 3) Get the offset difference for local and timezone
  const tzDate = new Date(thereLocaleStr);
  const diffHrs = (tzDate.getTime() - date.getTime()) / 1000 / 60 / 60;

  // 4) Use the difference to adjust the local offset
  const tzOffsetHrs = localOffsetHrs + diffHrs;

  return tzOffsetHrs;
}