Compensate clock skew 

Allows for a 3-minutes clock skew between systems in notBefore and notAfter attributes.

Errata E92
SAML Version 2.0 Errata 05. 01 May 2012. OASIS Approved Errata. http://docs.oasisopen.org/security/saml/v2.0/errata05/os/saml-v2.0-errata05-os.html