next_date.exs

def next %Date{year: y, month: m, day: d} do
  case Date.new(y, m, d + 1) do
    {:ok, nd} -> nd
    {:error, _} ->
      case Date.new(y, m + 1, 1) do
        {:ok, nd} -> nd
        {:error, _} ->
          {:ok, nd} = Date.new(y + 1, 1, 1)
          nd
      end
  end
end