Fix Calendar.strftime/2 format negative year incorrectly (#12268)

Closes #12266

Author: wingyplus

lib/elixir/lib/calendar.ex

@@ -791,7 +791,14 @@ defmodule Calendar do
 
   # Year
   defp format_modifiers("Y" <> rest, width, pad, datetime, format_options, acc) do
-    result = datetime.year |> Integer.to_string() |> pad_leading(width, pad)
+    {sign, year} =
+      if datetime.year < 0 do
+        {?-, -datetime.year}
+      else
+        {[], datetime.year}
+      end
+
+    result = [sign | year |> Integer.to_string() |> pad_leading(width, pad)]
     parse(rest, datetime, format_options, [result | acc])
   end
 

lib/elixir/test/elixir/calendar_test.exs

@@ -364,5 +364,12 @@ defmodule CalendarTest do
         Calendar.strftime(~D[2019-08-15], "%D", unknown: "option")
       end
     end
+
+    test "zero padding for negative year" do
+      assert Calendar.strftime(Date.new!(-1, 1, 1), "%Y") == "-0001"
+      assert Calendar.strftime(Date.new!(-11, 1, 1), "%Y") == "-0011"
+      assert Calendar.strftime(Date.new!(-111, 1, 1), "%Y") == "-0111"
+      assert Calendar.strftime(Date.new!(-1111, 1, 1), "%Y") == "-1111"
+    end
   end
 end