I do believe I've found out how to get a true delta time, and it doesn't even require an update.
http://dl.dropbox.com/u/10316081/truedt.capx
http://dl.dropbox.com/u/10316081/truedt/index.html
First number number is dt
Second is FPS generated from it
Third is the internal fps expression.
Click to add another circle.
EDIT: Would everyone be willing to test these in your browser?
"Get Time" Delta
http://dl.dropbox.com/u/10316081/truedt/index.html
"Get Milliseconds" Delta
http://dl.dropbox.com/u/10316081/truedttest/index.html
I can't offer the .capx for the second one. I had to monkey rig construct to make it.