Amazon API returns 'Request has expired'

[elliotthill]

{"$":{"xmlns":"http://ecs.amazonaws.com/doc/2013-08-01/"},"Error":[{"Code":["RequestExpired"],"Message":["Request has expired. Timestamp date is 2018-01-16T16:14:43.573Z."]}],"RequestID":["d8ee0968-8a27-4ce4-8933-55bc06ad6d7b"]}

When running code within a loop - if it goes over 15 minutes all subsequent requests fail with this message, even if you reinitialise the client.