Why does the counter created in javascript show different values?

There is a counter that counts down from 25. When I click on the link of the page, it continues to count from wherever it is left. So when I start at 25 and exit the page, it shows 15 when I come back 10 seconds later. But when I open it with a different browser, there is a different number, so the counter does not always show the same value. What could be the reason for this and what can I do?

var interval = 25000; 

      function reset() {
        localStorage.endTime = +new Date() + interval;
      }

      if (!localStorage.endTime) {
        reset();
      }
      function millisToMinutesAndSeconds(millis) {
        var seconds = ((millis % 60000) / 1000).toFixed(0);
        return (seconds < 10 ? "0" : "") + seconds;
       }
      setInterval(function () {
        var remaining = localStorage.endTime - new Date();
        if (remaining >= 0) {
          document.getElementById("timer").innerText =
            millisToMinutesAndSeconds(remaining);
        } else {
          reset();
        }
      }, 100);