Count-Up Javascript Code Question

Can someone explain how I can get the following count-up javascript code to include a comma when a number grows past three digits. Right now for example the code will only display 1000 instead of the proper 1,000.

Source for this code: http://www.javascriptkit.com/script/script2/countup.shtml

Thanks...

<script>
var montharray=new Array("Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec")

function countup(yr,m,d){
var today=new Date()
var todayy=today.getYear()
if (todayy < 1000)
todayy+=1900
var todaym=today.getMonth()
var todayd=today.getDate()
var todaystring=montharray[todaym]+" "+todayd+", "+todayy
var paststring=montharray[m-1]+" "+d+", "+yr
var difference=(Math.round((Date.parse(todaystring)-Date.parse(paststring))/(24*60*60*1000))*1)
difference+=" days"
document.write("It\'s been "+difference+" since the launch of JavaScript Kit!")
}
//enter the count up date using the format year/month/day
countup(1997,12,05)
</script>

Code (JavaScript):

 

I just looked up what function can do that and put it into your code:

https://jsfiddle.net/13k83mu7/

var montharray=new Array("Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec")

function countup(yr,m,d){
var today=new Date()
var todayy=today.getYear()
if (todayy < 1000)
todayy+=1900
var todaym=today.getMonth()
var todayd=today.getDate()
var todaystring=montharray[todaym]+" "+todayd+", "+todayy
var paststring=montharray[m-1]+" "+d+", "+yr
var difference=(Math.round((Date.parse(todaystring)-Date.parse(paststring))/(24*60*60*1000))*1)
difference+=" days"
function formatNumber (difference) {
    return difference.toString().replace(/(\d)(?=(\d{3})+(?!\d))/g, "$1,")
}
document.write("It\'s been "+formatNumber(difference)+" since the launch of JavaScript Kit!")
}
//enter the count up date using the format year/month/day
countup(1997,12,05)

Code (markup):

I am sure there are more efficient ways of doing the same thing.

 

Thank you very much the above code appears to have fixed my issue.