Use actual ISO8601 timestamps for masto API

Elixir uses an incompatible superset of ISO8601 that breaks some clients

lib/pleroma/web/common_api/utils.ex

@@ -328,9 +328,16 @@ def date_to_asctime(date) do
   end
 
   def to_masto_date(%NaiveDateTime{} = date) do
-    date
-    |> NaiveDateTime.to_iso8601()
-    |> String.replace(~r/(\.\d+)?$/, ".000Z", global: false)
+    # NOTE: Elixir’s ISO 8601 format is a superset of the real standard
+    # It supports negative years for example.
+    # ISO8601 only supports years before 1583 with mutual agreement
+    if date.year < 1583 do
+      ""
+    else
+      date
+      |> NaiveDateTime.to_iso8601()
+      |> String.replace(~r/(\.\d+)?$/, ".000Z", global: false)
+    end
   end
 
   def to_masto_date(date) when is_binary(date) do

test/pleroma/web/common_api/utils_test.exs

@@ -498,6 +498,14 @@ test "removes microseconds from date (String)" do
     test "returns empty string when date invalid" do
       assert Utils.to_masto_date("2015-01?23T23:50:07.123Z") == ""
     end
+
+    test "returns empty string when date is before the introduction of the Gregorian Calendar" do
+      assert Utils.to_masto_date("0621-01-01T00:00:00Z") == ""
+    end
+
+    test "returns empty string when date is BCE" do
+      assert Utils.to_masto_date("-420-01-01T00:00:00Z") == ""
+    end
   end
 
   describe "maybe_notify_mentioned_recipients/2" do