diff --git a/doc/lispref/commands.texi b/doc/lispref/commands.texi
index e37f6002430..a8ce294ad9a 100644
--- a/doc/lispref/commands.texi
+++ b/doc/lispref/commands.texi
@@ -2701,7 +2701,10 @@ the character at that position.
 @cindex timestamp of a mouse event
 @defun posn-timestamp position
 Return the timestamp in @var{position}.  This is the time at which the
-event occurred, in milliseconds.
+event occurred, in milliseconds.  Such a timestamp is reported
+relative to an arbitrary starting time that varies according to the
+window system in use.  On the X Window System, for example, it is the
+number of miliseconds since the X server was started.
 @end defun
 
   These functions compute a position list given particular buffer