When attempting to register the vSphere Replication appliance to the PSC, you may encounter this error:

Error
Server returned 'request expired' less than 0 seconds after request was issued, but it shouldn't have expired for at least 600 seconds.

On the appliance, look in the log file /opt/vmware/hms/logs/hms.log: if you see messages similar to the following, the time of the replication appliance and the PSC may not be in sync.

/opt/vmware/hms/logs/hms.log
2017-01-14 06:57:00.694 ERROR com.vmware.vim.sso.client.impl.SoapBindingImpl [main] (..client.impl.SoapBindingImpl) | SOAP faultjavax.xml.ws.soap.SOAPFaultException: The time now Wed Jan 18 12:27:31 MST 2017 does not fall in the request   ...