Save this file as /Library/LaunchDaemons/com.tjluoma.ntpdate.plist

com.tjluoma.ntpdate.plist

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
	<key>Label</key>
	<string>com.tjluoma.ntpdate</string>
	<key>ProgramArguments</key>
	<array>
		<string>/usr/sbin/ntpdate</string>
		<string>-b</string>
		<string>-u</string>
		<string>time.apple.com</string>
	</array>
	<key>RunAtLoad</key>
	<true/>
	<key>StandardErrorPath</key>
	<string>/private/var/log/ntpdate.log</string>
	<key>StandardOutPath</key>
	<string>/private/var/log/ntpdate.log</string>
	<key>StartCalendarInterval</key>
	<dict>
		<key>Hour</key>
		<integer>0</integer>
		<key>Minute</key>
		<integer>0</integer>
	</dict>
</dict>
</plist>

Needs to be owned by root:wheel and chmod 644