You would actually add those scripts to the system. Then, you could access them from any program.
At least that would seem to be the best method.
Many scripts can be used system-wide. And, then if you like, you could probably write a script that would perform the whole operation for you.
Perhaps assign a hot-key to a script that would use the first script to copy the time and date and then paste it.
With scripts, the flexibility is endless.
Another option is Automator. This one plugs-into your various apps and functions directly. You can fully customize your own functions with minimal time and a very low learning curve.
Here's some good articles on Automator to get you started:
http://www.macworld.com/topics/automator/index.php
http://www.macworld.com/2005/11/features/automatorhome/index.php
http://www.macworld.com/2006/04/secrets/junegeekfactor/index.php
http://www.macworld.com/2005/10/features/automatortrialrun/index.php
Once you understand how the program works, you'll find it capable of building pre-programmed tasks for everything from file operations, to Safari, to e-mail, iPhoto, and so on.
Hope this helps.