Handle years more than 9999 when printing, closes #8368

Author: José Valim

Diff for: lib/elixir/lib/calendar/date.ex

@@ -248,6 +248,8 @@ defmodule Date do
   Parses the extended "Dates" format described by
   [ISO 8601:2004](https://en.wikipedia.org/wiki/ISO_8601).
 
+  The year parsed by this function is limited to four digits.
+
   ## Examples
 
       iex> Date.from_iso8601("2015-01-23")

Diff for: lib/elixir/lib/calendar/datetime.ex

@@ -461,10 +461,9 @@ defmodule DateTime do
   As specified in the standard, the separator "T" may be omitted if
   desired as there is no ambiguity within this function.
 
-  Time representations with reduced accuracy are not supported.
-
-  Note that while ISO 8601 allows datetimes to specify 24:00:00 as the
-  zero hour of the next day, this notation is not supported by Elixir.
+  The year parsed by this function is limited to four digits and,
+  while ISO 8601 allows datetimes to specify 24:00:00 as the zero
+  hour of the next day, this notation is not supported by Elixir.
 
   ## Examples
 

Diff for: lib/elixir/lib/calendar/iso.ex

@@ -566,7 +566,7 @@ defmodule Calendar.ISO do
 
   defp zero_pad(val, count) when val >= 0 do
     num = Integer.to_string(val)
-    :binary.copy("0", count - byte_size(num)) <> num
+    :binary.copy("0", max(count - byte_size(num), 0)) <> num
   end
 
   defp zero_pad(val, count) do

Diff for: lib/elixir/lib/calendar/naive_datetime.ex

@@ -511,10 +511,9 @@ defmodule NaiveDateTime do
   As specified in the standard, the separator "T" may be omitted if
   desired as there is no ambiguity within this function.
 
-  Time representations with reduced accuracy are not supported.
-
-  Note that while ISO 8601 allows datetimes to specify 24:00:00 as the
-  zero hour of the next day, this notation is not supported by Elixir.
+  The year parsed by this function is limited to four digits and,
+  while ISO 8601 allows datetimes to specify 24:00:00 as the zero
+  hour of the next day, this notation is not supported by Elixir.
 
   ## Examples
 

Diff for: lib/elixir/test/elixir/calendar/iso_test.exs

@@ -35,6 +35,13 @@ defmodule Calendar.ISOTest do
     end
   end
 
+  describe "date_to_iso8601/4" do
+    test "handles years > 9999" do
+      assert Calendar.ISO.date_to_iso8601(10000, 1, 1, :basic) == "100000101"
+      assert Calendar.ISO.date_to_iso8601(10000, 1, 1, :extended) == "10000-01-01"
+    end
+  end
+
   defp iso_day_roundtrip(year, month, day) do
     iso_days = Calendar.ISO.date_to_iso_days(year, month, day)
     Calendar.ISO.date_from_iso_days(iso_days)