       true
 
   """
+  @doc since: "1.3.0"
   @spec leap_year?(year) :: boolean()
   @impl true
   def leap_year?(year) when is_integer(year) do

      4

  """
  @spec day_of_week(year, month, day) :: 1..7
  @impl true
  def day_of_week(year, month, day)

      31

  """
  @doc since: "1.4.0"
  @spec days_in_month(year, month) :: 28..31
  @impl true
  def days_in_month(year, month)
