offsetName(ts, opts) {
    throw new ZoneIsAbstractError();
  }
Returns the offset's common name (such as EST) at the specified timestamp
@abstract
@param {number} ts - Epoch milliseconds for which to get the name
@param {Object} opts - Options to affect the format
@param {string} opts.format - What style of offset to return. Accepts 'long' or 'short'.
@param {string} opts.locale - What locale to return the offset name in.
@return {string}