This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
Your code should not work, also not on 1.4, if the API documentation says that the object must be a Timestamp object.
If it does work on Java 1.4, then that's a bug in Java 1.4. If your program relies on this bug, then you should correct your program, because you don't want your program to rely on a bug in a particular Java version.