Simple countdown timer without the usage of setInterval

countdown.js

function sleep(delay) {
    var start = new Date().getTime();
    while (new Date().getTime() < start + delay) ;
}

function convertTime(hours) {
    var realHours = Math.floor(hours);
    var minutes = (hours - realHours) * 60;
    var realMinutes = Math.round(minutes);
    var realSeconds = ((hours - realHours) + (minutes - realMinutes)) * 60;
    var seconds = Math.round(realSeconds);

    if (seconds > 60) {
        realMinutes++;
        seconds -= 60;
    }

    return {hours: realHours, minutes: realMinutes, seconds: seconds}
}

function zeroPad(num, places) {
    var zero = places - num.toString().length + 1;
    return Array(+(zero > 0 && zero)).join("0") + num;
}

function printHours(hours, minutes, seconds) {
    return zeroPad(hours, 2) + ':' + zeroPad(minutes, 2) + ':' + zeroPad(seconds, 2);
}

function countDown(date, cb) {
    var now = new Date().getTime();

    if (now > date) {
        throw new Error('date is old');
    }

    var totalHours = Math.abs(now - date) / 3600000;

    var timeObj = convertTime(totalHours);
    var hours = timeObj.hours;
    var minutes = timeObj.minutes;
    var seconds = timeObj.seconds;

    cb(hours, minutes, seconds);

    while (hours > 0 || minutes > 0 || seconds > 0) {
        sleep(1000);

        seconds--;
        if (hours > 0 || minutes >= 0 || seconds >= 0) {

            if (hours > 0 && minutes === 0 && seconds < 0) {
                hours--;
                minutes = 59;
                seconds = 59;
            } else if (minutes > 0 && seconds < 0) {
                seconds = 59;
                minutes--;
            } else if ((hours > 0 || minutes > 0) && seconds < 0) {
                seconds = 59;
            }
        }

        cb(hours, minutes, seconds);
    }
}

try {
    var date = Date.parse("2019-03-25T17:00:00");
    countDown(date, function (hours, minutes, seconds) {
        console.log(printHours(hours, minutes, seconds));
    });
    console.log('countdown ended!')
} catch (e) {
    console.log(e);
}