If I understand correctly, this would be under the assumption that the movement is linear between the two frames. In reality the subject is most likely accelerating/decelerating but it could be a good enough approximation in many cases, better than nothing anyway.
Maybe this could be integrated into a line tool. You trace a line between two points that are supposed to be in two consecutive frames, and the line would have graduations corresponding to subdivisions of the frame interval, and show the corresponding time at each graduation.
Or a line with one sliding point in the middle, with a mini label attached, and you slide the point to where you are interested in reading the time.
Maybe called "time segment" tool, a segment of a timeline.