Most likely the problem is that the increment function has a different value of this than the other two functions. Try sticking console.log(this); in each function (then opening the dev console) to see the difference.
Simplest fix would probably be to pass this as a parameter from startPause, e.g. increment(this); Then use the passed value in the increment function.
As suggested above, the problem is due to incorrect reference to 'this' itself.
Either store 'this' on the main timeline as a different variable and use that variable in other functions in place of 'this', or
Replace this.timeDisplay.text with exportRoot.timeDisplay.text
Perfect! This is it! Now it is ticking. Thank you, Nipun!