Since I can't really find a game property to track current round time, you can use this snippet I created here:
https://gist.github.com/roflmuffin/bed001ee2d12cb49deff
It's in transitional syntax but it should be easy to understand. When the round starts we keep track of the time in a global variable, and when we want to know the time we just query the current time - when the round started.
I've put them in functions and included the example command for you. Just change 5 to however many seconds.