Convert a minute-based timezone offset like `240` to an offset like `"-04:00"`.

getTimezoneOffset.js

// Converts a getTimezoneOffset() offset to one that can be used in new Date().
// Examples:
// Eastern Standard Time: 240 -> '-04:00'
// India Standard Time: -330 -> '+05:30'
// Australian Central Western Standard Time: -525 -> '+08:45'
export function getTimezoneOffset(d) {
  return convertTimezoneOffset(d.getTimezoneOffset());
}

export function convertTimezoneOffset(offset) {
  const plusMinus = offset > 0 ? '-' : '+';
  const hours = Math.abs(Math.ceil(offset / 60)) + '';
  const minutes = (Math.abs(offset) % 60) + '';

  return `${plusMinus}${hours.padStart(2, '0')}:${minutes.padStart(2, '0')}`;
}

Usage examples:

convertTimezoneOffset(0);
'+00:00'
convertTimezoneOffset(240);
'-04:00'
convertTimezoneOffset(-330);
'+05:30'
convertTimezoneOffset(-525);
'+08:45'