I wanted to ensure that people were aware of this issue as it is causing a lot of incidents to come in to Product Support @ SAP.
It was recently discovered that a patch has introduced an issue with the calendar objects that are used by several Webi workflows. It seems that for for months that have 30 days or less, only 27 days will show up in the Calendar picker.
At the time of this writing, there is no solution. This does only affect the HTML viewer and scheduling workflows so you should be able to use Webi Rich Client and the Applet Viewer as a temporary work-around for now.