 
   # Converts year, month, day to count of days since 0000-01-01.
   @doc false
-  @doc since: "1.5.0"
   def date_to_iso_days(0, 1, 1) do
     0
   end


  # Converts count of days since 0000-01-01 to {year, month, day} tuple.
  @doc false
  @doc since: "1.5.0"
  def date_from_iso_days(days) when days in 0..3_652_424 do
    {year, day_of_year} = days_to_year(days)
    extra_day = if leap_year?(year), do: 1, else: 0

  end

  @doc false
  def iso_days_to_unit({days, {parts, ppd}}, unit) do
    day_microseconds = days * @parts_per_day
    microseconds = div(parts * @parts_per_day, ppd)
