It would be handy if the date that a user starts following another user is added to the API information.
The best place would probably be in the GET friendships/show API
We do not expose in the API when an account was followed. However, the numeric IDs returned by GET followers/ids and GET friends/ids methods are ordered (most recent first). You could easily guess the rough following date that way.
Are there any plans to expose it via the API?
I was rather hoping of getting the exact date than a guesstimate but if the data is not there then there is not much I can do about it.
I'm afraid we don't have any plan to expose this in a short term. But thanks for your feedback!
If you are using userstreams or have access to sitestreams, you could listen for the follow event that gets sent whenever a new follow happens. This would allow you to know when a follow occurred.