Date & Time from the toolbar (this is with Word XP). Next time you open up the document, you'll see today's date there.
Pick the specific date format you want, and make sure you click on the checkbox at the bottom that says Update Automatically. The only caveat is this: if you want to make sure you know when you saved a particular document, I suggest saving the document with the date in the filename because then next time you open up that document, the date will get updated.
Do this for all of the text you want to show up in the table of contents.
For example: If you are writing a book with chapters you could apply the Heading 1 style to each of your chapter titles.
After that, Word will build it automatically, from those headings.