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 a third party intercepting them (i.e., calls too far in the past or future will produce an error message). Required. If not provided, the error message 'E001: Parameter Timestamp is mandatory' appears. If the timestamp is too old or in the future, an error message "E003: Timestamp has expired" is returned.