The current time in ISO8601 format relative to UTC (e.g., Timestamp=2015-04-01T10:00:00+02:00 for Berlin), so that calls cannot be replayed by an eaves-dropping third party (i.e., those calls too far into the past or future yield an error message). Mandatory. If not supplied, an 'E001: Parameter Timestamp is mandatory' error message results. If the timestamp is too old or in the future, an 'E003: Timestamp has expired' error message is returned.