jquery.doomclock.js

$.fn.doomclock = function() {
  if (this[0].tagName === 'TIME') {
    var now = new Date(),
      date = new Date($(this).attr('datetime')),
      diff = (date - now) / 1000;

    if (diff > 0) {
      var countdown = {
        days: Math.floor(diff/86400),
        hours: ('0' + Math.floor(diff/3600) % 24).slice(-2),
        minutes: ('0' + Math.floor(diff/60) % 60).slice(-2),
        seconds: ('0' + Math.floor(diff % 60)).slice(-2)
      };

      this[0].innerHTML = '<span id="doomclock-days">' + countdown.days + '</span> days<br>' +
        '<span id="doomclock-hours">' + countdown.hours + '</span>:' +
        '<span id="doomclock-minutes">' + countdown.minutes + '</span>:' +
        '<span id="doomclock-seconds">' + countdown.seconds + '</span>';

      var days = document.getElementById('doomclock-days'),
        hours = document.getElementById('doomclock-hours'),
        minutes = document.getElementById('doomclock-minutes'),
        seconds = document.getElementById('doomclock-seconds');

      setInterval(function() {
        if (seconds.innerHTML == 0) {
          seconds.innerHTML = 59;
          if (minutes.innerHTML == 0) {
            minutes.innerHTML = 59;
            if (hours.innerHTML == 0) {
              hours.innerHTML = 23;
              if (days.innerHTML > 0) {
                --days.innerHTML;
              }
            } else {
              --hours.innerHTML;
              if (hours.innerHTML < 10) {
                hours.innerHTML = ('0' + hours.innerHTML).slice(-2);
              }
            }
          } else {
            --minutes.innerHTML;
            if (minutes.innerHTML < 10) {
              minutes.innerHTML = ('0' + minutes.innerHTML).slice(-2);
            }
          }
        } else {
          --seconds.innerHTML;
          if (seconds.innerHTML < 10) {
            seconds.innerHTML = ('0' + seconds.innerHTML).slice(-2);
          }
        }
      }, 1000);
    } else {
      this[0].innerHTML = "It's here!";
    }

  } else {
    throw "jQuery.doomclock: Invalid tagName. You must invoke doomclock on a <time> element.";
  }
}