There are several constants available in https://github.com/piwik/piwik/blob/2.16.0/core/Date.php#L43-L53 that can be used with getLocalized() I believe.

I think when adding a doc block with a long description see https://github.com/piwik/developer-documentation/blob/master/generator/template/pages/class.twig#L26 for those constants they should appear on http://developer.piwik.org/api-reference/Piwik/Date

It would be nice to have some examples for them in the doc re how the format will look like