javascript timestamp to relative time

1function timeDifference(current, previous) {
2
3    var msPerMinute = 60 * 1000;
4    var msPerHour = msPerMinute * 60;
5    var msPerDay = msPerHour * 24;
6    var msPerMonth = msPerDay * 30;
7    var msPerYear = msPerDay * 365;
8
9    var elapsed = current - previous;
10
11    if (elapsed < msPerMinute) {
12         return Math.round(elapsed/1000) + ' seconds ago';   
13    }
14
15    else if (elapsed < msPerHour) {
16         return Math.round(elapsed/msPerMinute) + ' minutes ago';   
17    }
18
19    else if (elapsed < msPerDay ) {
20         return Math.round(elapsed/msPerHour ) + ' hours ago';   
21    }
22
23    else if (elapsed < msPerMonth) {
24        return 'approximately ' + Math.round(elapsed/msPerDay) + ' days ago';   
25    }
26
27    else if (elapsed < msPerYear) {
28        return 'approximately ' + Math.round(elapsed/msPerMonth) + ' months ago';   
29    }
30
31    else {
32        return 'approximately ' + Math.round(elapsed/msPerYear ) + ' years ago';   
33    }
34}
35