diff --git a/reference/datetime/book.xml b/reference/datetime/book.xml
index 2a72a77ee2..9a193c1824 100644
--- a/reference/datetime/book.xml
+++ b/reference/datetime/book.xml
@@ -15,6 +15,10 @@
    where your PHP scripts are running. You can use these functions to format the
    date and time in many different ways.
   </para>
+  <para>
+   Each component of date (e.g. year) is internally stored as 64-bit number so
+   all imaginable dates (including negative years) are supported.
+  </para>
   <note>
    <simpara>
     Please keep in mind that these functions are dependent on