For completes, the USA keyboard layout works with:
Ctrl + Shift + : time.
Ctrl + ; date.
Ctrl + Alt + Shift + : is the heroic keystroke sequence to insert a datetime (time and date).
See Keyboard accelerators, mnemonics, and shortcuts for more details.
Came to this question from 2011 in 2021, and was still not clear why this no longer works Out of the Box!
It turns out that Google changed the defaults. The Keyboard Shortcuts are now off by default. I found out because I'm now using a (new) GSuite account for work.
So to turn it on go to Settings > General > Keyboard shortcuts and set it to Keyboard ...
As stated in an earlier comment there is no such shortcut by default.
Having said that and reading your answer: "I can create a new document by typing docs.new in the address bar", I believe you will be happier with the following.
How to build a custom shortcut for creating a new Google file
Steps for Google Chrome (a very popular browser)
Go to ...