gistfile1.js

alarmTime = "13:30"; // 1:30 pm

set_alarm = function() {
                  // Convert alarmTime to 24 format if it's 12 format
  var alarmCopy = (Options.timeFormat === '12') ? toggle_time_format(alarmTime) : alarmTime
    , time      = new Date()
    , cHour     = time.getHours()
    , cMin      = time.getMinutes()
                  // Extract hour and minute from alarmCopy string
    , aHour     = Number(alarmCopy.replace(/:[0-9]+/, ''))
    , aMin      = Number(alarmCopy.replace(/[0-9]+:/, ''))
    , millHour
    , millMin
    , tillAlarm;

  // This kind of works, but messes up horribly
  // - when the alarmTime is before the current time
  if(aHour < cHour) {
    millHour = ((24 - cHour) + aHour) * 3600000;
  } else millHour = (aHour - cHour) * 3600000;
  if(aMin < cMin) {
    millMin = ((60 - cMin) + aMin) * 600000;
  } else millMin = (aMin - cMin) * 600000;
  tillAlarm = millHour + millMin;

  clearInterval(alarmLoop);
  alarmLoop = undefined;

  alarmLoop = setTimeout(function() {
    alert("alarm");
  }, tillAlarm);
};