start ntpd with launchdaemon
There should be an ntpd process started by /System/Library/LaunchDaemons/org.ntp.ntpd.plist by default in 10.8.

The plist is also missing &lt;key&gt;RunAtLoad&lt;/key&gt;&lt;true/&gt;, but I couldn't get this to work either:

&lt;?xml version="1.0" encoding="UTF-8"?&gt;
&lt;!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN"
"http://www.apple.com/DTDs/PropertyList-1.0.dtd"&gt;
&lt;plist version="1.0"&gt;
&lt;dict&gt;
  &lt;key&gt;Label&lt;/key&gt;
  &lt;string&gt;com.stefan.ntpd&lt;/string&gt;
  &lt;key&gt;Program&lt;/key&gt;
  &lt;string&gt;/usr/sbin/ntpd&lt;/string&gt;
  &lt;key&gt;RunAtLoad&lt;/key&gt;
  &lt;true/&gt;
&lt;/dict&gt;
&lt;/plist&gt;