How to calculate "time ago" in Java?

Have you considered the TimeUnit enum? It can be pretty useful for this kind of thing

    try {
        SimpleDateFormat format = new SimpleDateFormat("dd/MM/yyyy");
        Date past = format.parse("01/10/2010");
        Date now = new Date();

        System.out.println(TimeUnit.MILLISECONDS.toMillis(now.getTime() - past.getTime()) + " milliseconds ago");
        System.out.println(TimeUnit.MILLISECONDS.toMinutes(now.getTime() - past.getTime()) + " minutes ago");
        System.out.println(TimeUnit.MILLISECONDS.toHours(now.getTime() - past.getTime()) + " hours ago");
        System.out.println(TimeUnit.MILLISECONDS.toDays(now.getTime() - past.getTime()) + " days ago");
    }
    catch (Exception j){
        j.printStackTrace();
    }

Take a look at the PrettyTime library.

It's quite simple to use:

import org.ocpsoft.prettytime.PrettyTime;

PrettyTime p = new PrettyTime();
System.out.println(p.format(new Date()));
// prints "moments ago"

You can also pass in a locale for internationalized messages:

PrettyTime p = new PrettyTime(new Locale("fr"));
System.out.println(p.format(new Date()));
// prints "à l'instant"

As noted in the comments, Android has this functionality built into the android.text.format.DateUtils class.