Clarify that due date emails are in the server's timezone

doc/user/project/issues/due_dates.md

@@ -36,7 +36,8 @@ Due dates also appear in your [todos list](../../../workflow/todos.md).
 ![Issues with due dates in the todos](img/due_dates_todos.png)
 
 The day before an open issue is due, an email will be sent to all participants
-of the issue.
+of the issue. Both the due date and the day before are calculated using the
+server's timezone.
 
 [ce-3614]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3614
 [permissions]: ../../permissions.md#project