Every RoR developer familiar with function distance_of_time_in_words, name of this function is self-explaining. I've been looking for similar function in AS3 world, I found few pieces of code on StackOverflow, but it was not what I needed. Eventually I've ported it on AS3 by myself. This function is already in use on twits info window.
Update: On Stack Overflow I found very useful class called TimeSpan. I just added my function to this class. Check it out.