When you say 'break down' what do you mean? Truncate it? Display it? Also what version of Tableau are you running? In 7 there are some options here:
It displays date like this:
Then there is the hour option here:
It displays the hour only.
If I knew a little more about what result you are looking for I might be able to help more.
Message was edited by: Shawn Wallwork
And Tableau customer support responded, convert the Date field to string and apply this calculation:
DATETIME(STR(DATE([Date])) + " " + RIGHT([Date], 2) + ":00:00")
This date format is what Omniture provides on their web analytics packages so worth bookmarking if you also visualize their data with Tableau.