For e.g. method=Referers.getRefererType it is no problem to get the statistics for
&date=previous1&period=month&format=xml (i.e. in xml format)

Same holds true for format=json.

However, this is not possible for tsv format with &date=previous1&period=month&format=tsv (same for format=csv). This gives the following error:

Error: Date format must be: YYYY-MM-DD, or 'today' or 'yesterday' or any keyword supported by the strtotime function (see http://php.net/strtotime for more information)

The API should be consistent and allow also csv/tsv format for this. Possibly, there are also other methods than Referers.getRefererType that have this behaviour.

Background: I would like to grab the previous month's statistics but not in xml but csv/tsv format. This feature request results from the discussion at http://forum.piwik.org/read.php?2,94478,94480