Addition/Subtraction of Erlang Date with built-in functions only

date_arith.ex

defmodule DateArith do
  def next_month({y, 12, d}), do: {y + 1, 1, d}
  def next_month({y, m,  d}), do: {y, m + 1, d}
  
  def previous_month({y, 1, d}), do: {y - 1, 12, d}
  def previous_month({y, m, d}), do: {y, m - 1, d}
  
  def add_month({y, m, d}, ms) do
    {
      y + div(m + ms - 1, 12),
      rem12(m + ms),
      d
    }
  end
  
  def sub_month(date, 0), do: date
  def sub_month({y, m, d}, ms) do
    {
      y + div(m - ms, 12) - 1,
      rem12(m - ms),
      d
    }
  end
  
  defp rem12(m) do
    rem = rem(m, 12)
    cond do
      rem == 0 -> 12
      rem <  0 -> rem + 12
      rem >  0 -> rem
    end
  end
end