SAML TP, Interop - dateTime formats

I found different dateTime formats for IssueInstant, NotOnOrAfter and
NotBefore timestamps in the SAML TP and Interop documents.

"2002-07-23T11:32:05.6228146-07:00" (SAML TP, Page 8, Line 226)
"2002-06-19T16:58:33.173Z"          (SAML TP, Page 15, Line 101)
="2003-04-17T00:46:02Z"             (Interop, Page 33, Line 928)

Is there a preferred/prescribed dateTime format? It would be nice to
have timestamps as defined in the WSS: SOAP Message Security 1.0->
Security Timestamps section (Page 36, 37).

--ms