How can I get very accurate time of a video interval?

let time1cmt = CMTimeGetSeconds(playerOK.currentTime())
time1 = Double(time1cmt)

let time2cmt = CMTimeGetSeconds(playerOK.currentTime())
time2 = Double(time2cmt)

videoDif = (time2 - time1)

Now I'm using .currentTime() to get the start point and end point, but they don't seem to be accurate, is there any other way to get the time more accurately?