toUTC(offset = 0, opts = {}) {
    return this.setZone(FixedOffsetZone.instance(offset), opts);
  }
"Set" the DateTime's zone to UTC. Returns a newly-constructed DateTime.

Equivalent to {@link DateTime#setZone}('utc')
@param {number} [offset=0] - optionally, an offset from UTC in minutes
@param {Object} [opts={}] - options to pass to `setZone()`
@return {DateTime}