Javascript: relative datetime

gistfile1.js

var relativeDate = function(date){
      if (navigator.appName === 'Microsoft Internet Explorer') return '';

      var unit = {
        now: 'Now',
        minute: '1 min',
        minutes: ' mins',
        hour: '1 hr',
        hours: ' hrs',
        day: 'Yesterday',
        days: ' days',
        week: '1 week',
        weeks: ' weeks'
      };

      var current = new Date(),
        tweet = new Date(date),
        diff = (((current.getTime() + (1 * 60000)) - tweet.getTime()) / 1000),
        day_diff = Math.floor(diff / 86400);

      if (day_diff == 0){
        if (diff < 60) return unit.now;
        else if (diff < 120) return unit.minute;
        else if (diff < 3600) return Math.floor(diff / 60) + unit.minutes;
        else if (diff < 7200) return unit.hour;
        else if (diff < 86400) return Math.floor(diff / 3600) + unit.hours;
        else return '';
      } else if (day_diff == 1) {
        return unit.day;
      } else if (day_diff < 7) {
        return day_diff + unit.days;
      } else if (day_diff == 7) {
        return unit.week;
      } else if (day_diff > 7) {
        return Math.ceil(day_diff / 7) + unit.weeks;
      } else {
        return '';
      }
    }

// Usage:
relativeDate(new Date())  // return "1 min"