Let's say original documentation for a class looks like this:
/**
* My custom class
*
* This class helps you do stuff. It's really great.
*
* @author David Smith
* @version 1.0
*/
If I come along and rework 50% of the code, how do I document that I made significant contributions?
What is the best way to add a date? e.g. When the code was last modified, when I made my updates, or when the version was updated.
I'm aware I'm not really answering your question, but I wonder why you're using @author at all.
For tracking changes to code (who created a file? who changed it?), use your version control system. That's what it's for, after all :)