Access messageExchange.timestamp in a groovy test script

targetStep = messageExchange.modelItem.testStep.testCase.getTestStepByName('Properties')
targetStep.setPropertyValue( 'timestamp', messageExchange.timestamp.toString())

From there I can access it in a subsequent script test step.

I forgot to mention that the 1st 2 rows must be placed in a script assertion of the response,
which you want to save the timestamp from.

Or is there an easier way ? The response itself does not contain a timestamp.
In a subsequent groovy script test step I need to compare the request/response timestamp against a database entry.