Noted 64-bit behaviour of PHP for timestamps.

git-svn-id: https://svn.php.net/repository/phpdoc/en/trunk@312829 c90b9560-bf6c-de11-be94-00142212c4b1

reference/datetime/functions/strtotime.xml

@@ -198,6 +198,11 @@ if (($timestamp = strtotime($str)) === false) {
     some Linux distributions, and a few other operating systems. PHP 5.1.0 and
     newer versions overcome this limitation though.
    </para>
+   <para>
+    For 64-bit versions of PHP, the valid range of a timestamp is effectively
+    infinite, as 64 bits can represent approximately 293 billion years in either
+    direction.
+   </para>
   </note>
   <note>
    <para>