Question

How can I write a script to detect when a user changes their system time in JS?

Answer 1

There is no (portable) way to track a variable in JavaScript. Also, date information does not lie in the DOM, so you don’t get the possibility of a DOM event being triggered.

The best you can do is to use setInterval to check periodically (every second?). Example:

function timeChanged(delta) {
  // Whatever
}

setInterval(function timeChecker() {
  var oldTime = timeChecker.oldTime || new Date(),
      newTime = new Date(),
      timeDiff = newTime - oldTime;

  timeChecker.oldTime = newTime;

  if (Math.abs(timeDiff) >= 5000) { // Five second leniency
    timeChanged(timeDiff);
  }
}, 500);