Based on this, you can get milliseconds by doing
OS.get_ticks_msec() % 1000
It should return a value of 0 - 999
but I'm not sure if this is the actual milliseconds or this is dependent on something else. Hope someone can affirm this is milliseconds.
Cons to this method of getting milliseconds is that you are basically asking the computer to do more math whenever you run this line. hope there will be one command to get milliseconds on godot 4.0