1. The Idea
Counting overall time would be awesome already. In order to avoid overloading the servers, you could just count every 5, 10 hours or something like that.
As a plus (I have no idea how hard this is, please forgive me), there could be graphs showing how much time you spent by day, week, month, year, etc.
This would be a good way to help Tilders prevent Tildes addiction (on the other hand, there would have to be some mechanic to prevent people from gamyfying this...).
This time count would be entirely private to each user by default, but there would the option to display it.
2. The Reason
I find that knowing how much time I spent on something helps me be more productive. It's the entire reason for the existence of tools such Rescue Time, Waka Time and Toggl. If I know how much I spend on a platform, it's easier to control my addiction to it. These other tools are heavy and focused on billing hours. That's not my use case. I know this might seem silly, but it makes a lot of difference for some people. 7.2% of the world's population has ADHD. I happen to be one of them.