function defineComparators(a, b, unit) {
    const datePart = fixUnit(unit)
    const dtA = datePart ? moment(a).startOf(datePart) : moment(a)
    const dtB = datePart ? moment(b).startOf(datePart) : moment(b)
    return [dtA, dtB, datePart]
  }
BEGIN localized date arithmetic methods with moment **