you can't count on any timing in flash being ms-accurate.
what you see is dependent on your local computer system. what another user of your swf sees is dependent on their local system so, even if you get timing accurate to within 10ms on your system, it does not mean anyone else will see the same.
that said, increasing your frame rate and using the timer class (as3) or setInterval (as2) will generate the quickest loops suitable for timing and using getTimer() is more accurate than the timer/setInterval delay parameter.
i tried setting the frame rate faster (1000Hz), but it maxed out at the monitors frame rate (60Hz) - which kinda explains the resolution of Date.time
the high-speed timer only has to be consistent on my system. i'm using the timer for performance testing and won't need it in a "shipping" environment.
then set your frame rate to 120, don't put anything on stage and use the timer class or setInterval() with getTimer() to do whatever you want.