Special Twitter forensics can reveal the exact day a Twitter follow action took place.

According to ye wondrous interwebz, this can’t be done. But that’s never stopped me in the past. In yet another advanced Twitter forensics first, I’m going to show you, here and now, how to extract actual follow date information from a Twitter follow list. In other words, when your followers followed you. Or when you followed your followers. Or when someone else’s followers followed them.
[UPDATE:2021… In December 2020, Twitter shut down the Legacy Mobile environment, taking the method described in this post with it. If I can find a new method I will update the post, but for now, everything below this paragraph remains for retrospective interest only.]
The app you'll be using is just below, for ease of future access. But if you haven't used it before you'll need to read the rest of the post first…
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Before we get started, I should stress that Twitter does not make this easy. There are multiple steps to the process, and it is an advanced level technique. However, you don't need to be Einstein to do this. There are pictures in the tutorial, and I've made a converter app so you don't have to do any 'workings out'. Once you've learned the process, it's something you'll probably want to keep using, as it's a completely new world of exploration which puts so much previously unseen info at your fingertips. The method is well worth learning in my opinion.