set until time "to_date('03/29/2010 08:00:00','MM/DD/YYYY HH24:MI:SS')";
However, most of the time I just want to have my data as of an hour ago no matter what time I start the clone and don't want to edit the timestamp each time (less stuff to mess up). Apparently this syntax also works.
You can take this a step further in complication with anything for dates, such as cloning to the most recent archivelog backup (assuming a scheduled archive log backup takes place every hour at 35 after)
set until time "sysdate - 1/24";
set until time "trunc((sysdate - 30/1440),'HH24')+30/1440";