After some investigation from the dojo source and the online material from http://livedocs.dojotoolkit.org/dijit/form/DateTextBox
it seems as though the date that the pop up focuses on is bound to the value that is set in the edit box and seems to have no way to set a "currentFocus" in the same way as just using the dijit.calendar. Only setting the value or the default value causes the focus to shift from the current date.
If your use case requires someone to actually select a date perhaps you could check if the date text box has been clicked instead of checking its value?