Complete DateDiff() Function

Perhaps the above function looks like the Visual Basic function of the same name. In fact it is loosely based on it. I was planning on recreating it in its complete form for your enjoyment, but, thankfully someone has already beat me to it. That someone is Rob Eberhardt of Slingshot Solutions. It's part of his excellent jsDate script. It's free to use as long as you give credit where credit is due. His function offers a lot of advantages over the simple one presented above. For starters, his can calculate the month interval, which cannot be done by dividing into the number of milliseconds since month lengths differ. It also supports setting the first day of the week to something other than Sunday. Finally, it adjusts for Daylight Savings Time, which affects intervals of a day ("d") and larger: 
Date.DateDiff = function(p_Interval, p_Date1, p_Date2, p_FirstDayOfWeek){
 p_FirstDayOfWeek = (isNaN(p_FirstDayOfWeek) || p_FirstDayOfWeek==0) ? vbSunday : parseInt(p_FirstDayOfWeek);

 var dt1 = Date.CDate(p_Date1);
 var dt2 = Date.CDate(p_Date2);

 //correct Daylight Savings Ttime (DST)-affected intervals ("d" & bigger)
 if("h,n,s,ms".indexOf(p_Interval.toLowerCase())==-1){
  if(p_Date1.toString().indexOf(":") ==-1){ dt1.setUTCHours(0,0,0,0) }; // no time, assume 12am
  if(p_Date2.toString().indexOf(":") ==-1){ dt2.setUTCHours(0,0,0,0) }; // no time, assume 12am
 }


 // get ms between UTC dates and make into "difference" date
 var iDiffMS = dt2.valueOf() - dt1.valueOf();
 var dtDiff = new Date(iDiffMS);

 // calc various diffs
 var nYears  = dt2.getUTCFullYear() - dt1.getUTCFullYear();
 var nMonths = dt2.getUTCMonth() - dt1.getUTCMonth() + (nYears!=0 ? nYears*12 : 0);
 var nQuarters = parseInt(nMonths / 3);
 
 var nMilliseconds = iDiffMS;
 var nSeconds = parseInt(iDiffMS / 1000);
 var nMinutes = parseInt(nSeconds / 60);
 var nHours = parseInt(nMinutes / 60);
 var nDays  = parseInt(nHours / 24); //now fixed for DST switch days
 var nWeeks = parseInt(nDays / 7);

 if(p_Interval.toLowerCase()=='ww'){
   // set dates to 1st & last FirstDayOfWeek
   var offset = Date.DatePart("w", dt1, p_FirstDayOfWeek)-1;
   if(offset){ dt1.setDate(dt1.getDate() +7 -offset); }
   var offset = Date.DatePart("w", dt2, p_FirstDayOfWeek)-1;
   if(offset){ dt2.setDate(dt2.getDate() -offset); }
   // recurse to "w" with adjusted dates
   var nCalWeeks = Date.DateDiff("w", dt1, dt2) + 1;
 }
 
 // return difference
 switch(p_Interval.toLowerCase()){
  case "yyyy": return nYears;
  case "q": return nQuarters;
  case "m": return nMonths;
  case "y": // day of year
  case "d": return nDays;
  case "w": return nWeeks;
  case "ww":return nCalWeeks; // week of year 
  case "h": return nHours;
  case "n": return nMinutes;
  case "s": return nSeconds;
  case "ms":return nMilliseconds;
  default : return "invalid interval: '" + p_Interval + "'";
 }
}
var y2k  = new Date(2000, 0, 1) //Month is 0-11 in JavaScript!
var today= new Date();
console.log('Months since the new millenium: ' + Date.DateDiff('m', y2k, today)); //displays 143
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