How to use the System time inside a proxy service

system time can be used inside a proxy service. Following proxy uses the system time.

<proxy xmlns="http://ws.apache.org/ns/synapse" name="Proxy2" transports="https,http" statistics="disable" trace="disable" startOnLoad="true">
   <target>
      <inSequence>
         <log level="custom">
            <property name="system-time" expression="get-property('SYSTEM_DATE', 'HH:MM:SS')" />
         </log>
      </inSequence>
      <outSequence>
         <send />
      </outSequence>
      <endpoint>
         <address uri="http://localhost:8280/services/Version" />
      </endpoint>
   </target>
   <publishWSDL uri="http://localhost:8280/services/Version?wsdl" />
</proxy>