lib/pleroma/web/common_api/utils.ex

@@ -328,20 +328,27 @@ defmodule Pleroma.Web.CommonAPI.Utils do
   end
 
   def to_masto_date(%NaiveDateTime{} = date) do
-    date
-    |> NaiveDateTime.to_iso8601()
-    |> String.replace(~r/(\.\d+)?$/, ".000Z", global: false)
+    # NOTE: Elixir’s ISO 8601 format is a superset of the real standard
+    # It supports negative years for example.
+    # ISO8601 only supports years before 1583 with mutual agreement
+    if date.year < 1583 do
+      "1970-01-01T00:00:00Z"
+    else
+      date
+      |> NaiveDateTime.to_iso8601()
+      |> String.replace(~r/(\.\d+)?$/, ".000Z", global: false)
+    end
   end
 
   def to_masto_date(date) when is_binary(date) do
     with {:ok, date} <- NaiveDateTime.from_iso8601(date) do
       to_masto_date(date)
     else
-      _ -> ""
+      _ -> "1970-01-01T00:00:00Z"
     end
   end
 
-  def to_masto_date(_), do: ""
+  def to_masto_date(_), do: "1970-01-01T00:00:00Z"
 
   defp shortname(name) do
     with max_length when max_length > 0 <-

test/pleroma/web/common_api/utils_test.exs

@@ -495,8 +495,16 @@ defmodule Pleroma.Web.CommonAPI.UtilsTest do
       assert Utils.to_masto_date("2015-01-23T23:50:07.123Z") == "2015-01-23T23:50:07.000Z"
     end
 
-    test "returns empty string when date invalid" do
-      assert Utils.to_masto_date("2015-01?23T23:50:07.123Z") == ""
+    test "returns unix epoch when date invalid" do
+      assert Utils.to_masto_date("2015-01?23T23:50:07.123Z") == "1970-01-01T00:00:00Z"
+    end
+
+    test "returns unix epoch when date is before the introduction of the Gregorian Calendar" do
+      assert Utils.to_masto_date("0621-01-01T00:00:00Z") == "1970-01-01T00:00:00Z"
+    end
+
+    test "returns unix epoch when date is BCE" do
+      assert Utils.to_masto_date("-0420-01-01T00:00:00Z") == "1970-01-01T00:00:00Z"
     end
   end