How to use variables in file templates

I managed to figure out that when you use $<sometext> in filetemplates, you are presented with a dialog box when creating a new file of that type where you can fill in a value for this variable.
I also found (by accident) that ${USER} automatically fills in the user name.

What other built-in variables are there?  I would like to automatically fill in things like:

-current date
-last modified date
-client name

Is this possible?