You can find a list of built-in page fields here: http://processwire.com/api/variables/page/. As you can see, "date" is not among them. What you seem to want here is "created."
If you need the value of that field to be easily editable later, you could also create your own date field and apply some magic to that -- check "Default to today's date", hide field and show when needed / show always and handle pages with date value cleared somehow etc.
Hope this helps!