Trouble with Timer using Ticks

I am having trouble getting a timer function to work. I have attached my current code and bound the seconds and minutes variable to a UI text widget. However, I am confused on how to record milliseconds. The project is built around the very precision of recording a milliseconds variable so it is vital to the core mechanics. I could likely figure out the code myself if I could find documentation on how long a tick is. What do I add to this code to measure milliseconds. Thanks!