formatOffset(ts, format) {
    throw new ZoneIsAbstractError();
  }
Returns the offset's value as a string
@abstract
@param {number} ts - Epoch milliseconds for which to get the offset
@param {string} format - What style of offset to return.
                         Accepts 'narrow', 'short', or 'techie'. Returning '+6', '+06:00', or '+0600' respectively
@return {string}