To add to what Indumon said, if you want to be able to display different formats given different date aggregation selections, (via a parameter) then I recommend something like what's referenced in this thread: http://community.tableau.com/thread/129821
e.g.,If you select "Year", you get 2012, 2013, etc.
If you select "Month", you get Nov-12, Dec-12, Jan-13, etc.
If you select "Day", you get 11/30/2012, 12/1/2012, 12/2,2012, etc.
Thanks so much, Indumon. This got me on the right track and Mark's answer helped me get the axis labels correct.