You could set the display in the viewer or canvas to frames instead of timecode, and go to the location you want to calculate milliseconds for.
Then look up item properties for the video element, and get the frame rate for the video.
Then your calculation could be [(1 / frame rate) * frame number] * 1000 (milliseconds).
So frame number 2997, with a frame rate of 29.97 frames per second, would be (1 / 29.97) * 2997 * 1000 = .0333667 * 2997 * 1000 or 100,000 milliseconds (100 seconds).