Check out example codes for "unity timer with miliseconds". It will help you in understanding the concepts better.

Code Example 1

string FormatTime (float time){
         int intTime = (int)time;
         int minutes = intTime / 60;
         int seconds = intTime % 60;
         float fraction = time * 1000;
         fraction = (fraction % 1000);
         string timeText = String.Format ("{0:00}:{1:00}:{2:000}", minutes, seconds, fraction);
         return timeText;
     }

Learn ReactJs, React Native from akashmittal.com