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