flash actionscript: This is my code in in the actions layer

as you can see it should update every second and it does because when I do a
trace it shows in the output box that the counter increases. The problem is
that I also have a dynamic text field with the var = cliptime and the instance
= cliptime. When the code loads I do get it to output "0:00" but as the movie
increments the text box doesn't change at all.

setInterval(displayTime, 600);
import mx.video.*

function displayTime():Void{
n = Math.round(my_movie.playheadTime);
mins = Math.floor(n/60);
secs = n - 60*mins;
secs<10 ? secs = "0"+secs : secs;
cliptime.text = mins+":"+secs
trace(mins+":"+secs)
}