Reparse time to ensure correct format.

lib/pleroma/web/mastodon_api/mastodon_api_controller.ex

@@ -158,7 +158,7 @@ defmodule Pleroma.Web.MastodonAPI.MastodonAPIController do
     result = Enum.map(notifications, fn (%{id: id, activity: activity, inserted_at: created_at}) ->
       actor = User.get_cached_by_ap_id(activity.data["actor"])
       created_at = NaiveDateTime.to_iso8601(created_at)
-      |> String.replace(~r/\.\d+Z/, ".000Z")
+      |> String.replace(~r/\.\d+$/, ".000Z")
       case activity.data["type"] do
         "Create" ->
           %{id: id, type: "mention", created_at: created_at, account: AccountView.render("account.json", %{user: actor}), status: StatusView.render("status.json", %{activity: activity})}

lib/pleroma/web/mastodon_api/views/status_view.ex

@@ -27,7 +27,9 @@ defmodule Pleroma.Web.MastodonAPI.StatusView do
     attachments = render_many(object["attachment"] || [], StatusView, "attachment.json", as: :attachment)
 
     created_at = (object["published"] || "")
-    |> String.replace(~r/\.\d+Z/, ".000Z")
+    |> NaiveDateTime.from_iso8601!
+    |> NaiveDateTime.to_iso8601
+    |> String.replace(~r/\.\d+$/, ".000Z")
 
     %{
       id: activity.id,