WS-Security defines the Security SOAP header to carry security information in SOAP messages. Information included in this element includes, but not limited to, authentication credentials, digital signatures, and encryption references.  To specify security information for intermediary processing, use the actor element on the Security SOAP header.  WS-Security specifies the wsu:Timestamp element as a child of the wsse:Security header.

<soap:Header>
   ...
   <wsse:Security>
      <wsu:Timestamp>
         <wsu:Created>2003-06-04T03:48:32Z</wsu:Created>
         <wsu:Expires>2003-06-04T03:53:32Z</wsu:Expires>
      </wsu:Timestamp>
      ...
   </wsse:Security>
   ...
</soap:Header>