The following info seems to be misleading and has a bug, unless I am totally missing the logic here.
Example: (From NAnt doc)
Set the build.date property.
<tstamp property="build.date" pattern="yyyyMMdd" verbose="true" />
No matter what pattern you use. The task returns the same output as below...
Target framework: Microsoft .NET Framework 2.0
[tstamp] Tuesday, April 13, 2010 9:46:38 AM.
Total time: 0 seconds.