javascript如何计算两个日期之间的时间间隔

<script>
function NewDate(str)
{ 
  str=str.split('-'); 
  var date=new Date(); 
  date.setUTCFullYear(str[0], str[1] - 1, str[2]); 
  date.setUTCHours(0, 0, 0, 0); 
  return date; 
} 
function TimeCom(dateValue) 
{ 
  var newCom; 
  if (dateValue == "") 
  { 
    newCom = new Date(); 
  } 
  else
  { 
    newCom = NewDate(dateValue); 
  } 
  this.year = newCom.getYear(); 
  this.month = newCom.getMonth() + 1; 
  this.day = newCom.getDate(); 
  this.hour = newCom.getHours(); 
  this.minute = newCom.getMinutes(); 
  this.second = newCom.getSeconds(); 
  this.msecond = newCom.getMilliseconds(); 
  this.week = newCom.getDay(); 
} 
function DateDiff(interval, date1, date2) 
{ 
  var TimeCom1 = new TimeCom(date1); 
  var TimeCom2 = new TimeCom(date2); 
  var result; 
  switch (String(interval).toLowerCase()) 
  { 
    case "y": 
    case "year": 
    result = TimeCom1.year - TimeCom2.year; 
    break; 
    case "m": 
    case "month": 
    result = (TimeCom1.year - TimeCom2.year) * 12 + (TimeCom1.month - TimeCom2.month); 
    break; 
    case "d": 
    case "day": 
    result = Math.round((Date.UTC(TimeCom1.year, TimeCom1.month - 1, TimeCom1.day) - Date.UTC(TimeCom2.year, TimeCom2.month - 1, TimeCom2.day)) / (1000 * 60 * 60 * 24)); 
    break; 
    case "h": 
    case "hour": 
    result = Math.round((Date.UTC(TimeCom1.year, TimeCom1.month - 1, TimeCom1.day, TimeCom1.hour) - Date.UTC(TimeCom2.year, TimeCom2.month - 1, TimeCom2.day, TimeCom2.hour)) / (1000 * 60 * 60)); 
    break; 
    case "min": 
    case "minute":result = Math.round((Date.UTC(TimeCom1.year, TimeCom1.month - 1, TimeCom1.day, TimeCom1.hour, TimeCom1.minute) - Date.UTC(TimeCom2.year, TimeCom2.month - 1, TimeCom2.day, TimeCom2.hour, TimeCom2.minute)) / (1000 * 60)); 
    break; 
    case "s": 
    case "second":result = Math.round((Date.UTC(TimeCom1.year, TimeCom1.month - 1, TimeCom1.day, TimeCom1.hour, TimeCom1.minute, TimeCom1.second) - Date.UTC(TimeCom2.year, TimeCom2.month - 1, TimeCom2.day, TimeCom2.hour, TimeCom2.minute, TimeCom2.second)) / 1000); 
    break; 
    case "ms": 
    case "msecond": result = Date.UTC(TimeCom1.year, TimeCom1.month - 1, TimeCom1.day, TimeCom1.hour, TimeCom1.minute, TimeCom1.second, TimeCom1.msecond) - Date.UTC(TimeCom2.year, TimeCom2.month - 1, TimeCom2.day, TimeCom2.hour, TimeCom2.minute, TimeCom2.second, TimeCom1.msecond); 
    break; 
    case "w": 
    case "week":result = Math.round((Date.UTC(TimeCom1.year, TimeCom1.month - 1, TimeCom1.day) - Date.UTC(TimeCom2.year, TimeCom2.month - 1, TimeCom2.day)) / (1000 * 60 * 60 * 24)) % 7; 
    break; 
    default: 
    result = "invalid"; 
  } 
  return (result); 
}
</script>