The comment from @martineau is correct.
Here's what I did:
Download the 1.5 package from http://labix.org/python-dateutil (1.5 because you want it compatible with Python 2.x)
Copy the dateutil directory from the archive to the Notepad++ directory,
plugins\PythonScript\lib
- this is probablyC:\Program Files\Notepad++\plugins\PythonScript\lib
, orC:\Program Files (x86)\Notepad++\plugins\PythonScript\lib
, but could be elsewhere. Look at where Notepad++ is installed if you're not sure.import dateutil now works, and you can use the various features.
Full disclosure: I'm the author of PythonScript