I've been looking for a way to know the date two users became friends. I've found this question but it only explains how to simulate the friendship page with multiple FQL queries.
What I want to determine is the date you can see at the top of the friendship page ("Facebook friends since...").
This date isn't available via the Graph API or FQL tables. You can log a feature request with Facebook or get creative with workarounds such as those you linked in the post.