//Set the two dates
var millennium =new Date(1981, 0, 1) //Month is 0-11 in JavaScript
today=new Date()
//Get 1 day in milliseconds
var one_day=1000*60*60*24*365.25

//Calculate difference btw the two dates, and convert to days
document.write(Math.ceil((today.getTime()-millennium.getTime())/(one_day))+
" years.")