∀ {leap : Bool} (ordinal : Std.Time.Day.Ordinal.OfYear leap) (idx : Std.Time.Month.Ordinal) (acc : ℤ),
  ↑ordinal > acc →
    ↑ordinal ≤ acc + ↑(Std.Time.Month.Ordinal.days leap idx) →
      acc + 1 ≤ ↑ordinal ∧ ↑ordinal ≤ acc + ↑(Std.Time.Month.Ordinal.days leap idx)