   @impl true
   def day_of_week(year, month, day)
       when is_integer(year) and is_integer(month) and is_integer(day) do
-    :calendar.day_of_the_week(year, month, day)
+    Integer.mod((date_to_iso_days(year, month, day) + 5), 7) + 1
   end
 
   @doc """


  @impl true
  def valid_date?(year, month, day) do
    year <= 9999 and :calendar.valid_date(year, month, day)
  end

  @impl true

  @seconds_per_hour 60 * 60
  @seconds_per_day 24 * 60 * 60 # Note that this does _not_ handle leap seconds.
  @microseconds_per_second 1_000_000

  @days_per_nonleap_year 365
  @days_per_leap_year 366

  @doc """
  Returns the `t:Calendar.iso_days` format of the specified date.
