/Library/LaunchDaemon/ntpdate.plist

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>ntpdate</string>
    <key>Disabled</key>
        <false/>
    <key>UserName</key>
        <string>root</string>
    <key>RunAtLoad</key>
        <true/>
    <key>KeepAlive</key>
        <false/>
    <key>LaunchOnlyOnce</key>
        <true/>
    <key>ProgramArguments</key>
        <array>
                <string>/usr/sbin/ntpdate</string>
                <string>time.nist.gov</string>
        </array>
</dict>
</plist>