Display time based on frames

I have a movie clip and i would like to display its duration in minutes and seconds. currently I am displaying the frames so it shows “frame 320 of 500” etc.
I would like to figure out how many frames in the movie clip and convert it to time so in the end it will display: 1:23 / 3:00

frameNumber = "frame " + getProperty(movie, _currentframe) + " of " + getProperty(movie, _totalframes);
gotoAndPlay(_currentframe - 1);