diff --git a/CHANGELOG b/CHANGELOG
index a52957d45..b87e18bfd 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -218,6 +218,10 @@ o In the interest of forward compatibility, the xmloutputversion
 
 o Zenmap no longer leaves any temporary files lying around. [David]
 
+o Nmap only prints an uptime guess in verbose mode now, because in some
+  situations it can be very inaccurate. See the discussion at
+  http://seclists.org/nmap-dev/2008/q3/0392.html.
+
 Nmap 4.68 [2008-6-28]
 
 o Doug integrated all of your version detection submissions and
diff --git a/docs/refguide.xml b/docs/refguide.xml
index 6bc274ae2..c68570f5b 100644
--- a/docs/refguide.xml
+++ b/docs/refguide.xml
@@ -108,7 +108,6 @@ PORT    STATE  SERVICE VERSION
 Device type: general purpose
 Running: Linux 2.4.X|2.5.X|2.6.X
 OS details: Linux 2.4.7 - 2.6.11, Linux 2.6.0 - 2.6.11
-Uptime 33.908 days (since Thu Jul 21 03:38:03 2005)
 
 Interesting ports on playground.nmap.org (192.168.0.40):
 (The 1659 ports scanned but not shown below are in state: closed)
@@ -1872,10 +1871,9 @@ way.</para>
     contribute to the pool of operating systems known to Nmap and thus
     it will be more accurate for everyone.</para>
 
-    <para>OS detection enables several other tests which make use
-    of information that is gathered during the process anyway.  One of these
-    is uptime measurement, which uses the TCP timestamp option (<ulink role="hidepdf" url="http://www.rfc-editor.org/rfc/rfc1323.txt">RFC 1323</ulink>) to guess when a machine was last rebooted.  This is only
-    reported for machines which provide this information.  Another is
+    <para>OS detection enables some other tests which make use
+    of information that is gathered during the process anyway.
+    One of these is
     TCP Sequence Predictability Classification.  This measures
     approximately how hard it is to establish a forged
     TCP connection against the remote host.  It is useful for
@@ -1894,6 +1892,15 @@ way.</para>
     vulnerable to several advanced information gathering and
     spoofing attacks.</para>
 
+    <indexterm><primary>uptime guess</primary></indexterm>
+    <para>
+    Another bit of extra information enabled by OS detection is a guess
+    at a target's uptime. This uses the TCP timestamp option (<ulink role="hidepdf" url="http://www.rfc-editor.org/rfc/rfc1323.txt">RFC 1323</ulink>) to guess when a machine was last rebooted.
+    The guess can be inaccurate due to the timestamp counter not being
+    initialized to zero or the counter overflowing and wrapping around,
+    so it is printed only in verbose mode.
+    </para>
+
     <para>
     <man>
     A paper documenting the workings, usage, and customization of OS
diff --git a/output.cc b/output.cc
index e85e987b7..12fe9191f 100644
--- a/output.cc
+++ b/output.cc
@@ -1738,7 +1738,8 @@ void printosscanoutput(Target *currenths) {
     gettimeofday(&tv, NULL);
     strncpy(tmbuf, ctime(&(currenths->seq.lastboot)), sizeof(tmbuf));
     chomp(tmbuf);
-    log_write(LOG_PLAIN,"Uptime: %.3f days (since %s)\n", (double) (tv.tv_sec - currenths->seq.lastboot) / 86400, tmbuf);
+    if (o.verbose)
+      log_write(LOG_PLAIN,"Uptime guess: %.3f days (since %s)\n", (double) (tv.tv_sec - currenths->seq.lastboot) / 86400, tmbuf);
     log_write(LOG_XML, "<uptime seconds=\"%li\" lastboot=\"%s\" />\n", tv.tv_sec - currenths->seq.lastboot, tmbuf);
   }