Does SAP PO support time skew?

Hello Experts,

While using AXIS adapter, WS-Security header can be configured for actions UsernameToken and Timestamp.

Timestamp format is like:

<wsu:Timestamp wsu:Id="TS-889">
     <wsu:Created>2014-06-10T05:38:20.179Z</wsu:Created>
     <wsu:Expires>2014-06-10T06:08:20.179Z</wsu:Expires>

</wsu:Timestamp>

1. I have observed that SOAP requests with "Created" in the future timestamp is accepted as valid request. Why so?
2. In case of any difference between the clocks of "Sender" and SAP PO, time skew (if configured) will take care of the variations in the clock. I believe it should be configured at system level and by system admin. Correct?
3. Is there any document relating to how time skew is supported by SAP PO?

Thanks,

Ankit