ISO 8601 format to satisfy xs:datetime as interpreted by EWS. Examples:
            2009-01-15T13:45:56Z
            2009-01-15T13:45:56+01:00

def ewsformat(self):
        """
        ISO 8601 format to satisfy xs:datetime as interpreted by EWS. Examples:
            2009-01-15T13:45:56Z
            2009-01-15T13:45:56+01:00
        """
        if not self.tzinfo:
            raise ValueError('EWSDateTime must be timezone-aware')
        if self.tzinfo.zone == 'UTC':
            return self.strftime('%Y-%m-%dT%H:%M:%SZ')
        return self.replace(microsecond=0).isoformat()