diff --git a/reference/datetime/functions/date-default-timezone-get.xml b/reference/datetime/functions/date-default-timezone-get.xml
index 475ebff27e..2ff0ca7c0f 100644
--- a/reference/datetime/functions/date-default-timezone-get.xml
+++ b/reference/datetime/functions/date-default-timezone-get.xml
@@ -35,7 +35,11 @@
     </listitem>
     <listitem>
      <para>
-      Querying the host operating system (if supported and allowed by the OS)
+      Querying the host operating system (if supported and allowed by the OS). This uses an algorithm
+      that has to <strong>guess</strong> the timezone. This is by no means going to work correctly for 
+      every situation. A warning is shown when this stage is reached. Do not rely on it to be guessed
+      correctly, and set <link linkend="ini.date.timezone">date.timezone</link> to the correct timezone
+      instead.
      </para>
     </listitem>
    </itemizedlist>