> I just haven't seen it being used in such a way (adding an older date)

Yea, it's not a common practice, as far as I'm aware. I see it as a little experiment on a small data set which can be extended to the broader range of places, if successful. I remember someone forking our infra and pointing it to the tourist attractions data set or something =)

> Are you using this for BTCMap in some way?

Not yet. I maintain an Android app and it does display timestamps (max(survey:date, check_date, check_date:currency:XBT)), but it doesn't warn the users of obsolete locations. It only shows warnings for the places without any timestamps (~800 places, about 8% of the data set). I think it might help to warn the users about the obsolete or unreliable timestamps, but the current quality baseline is not high enough to raise the bar yet.

There are three numbers we're watching when evaluating our impact:

1. Total places (about 10k now)
2. Up-to-date places (checked less than a year ago, ~65% currently)
3. Average days since verified (~400, if I remember correctly)

In my view, the third number is the most important one. If it starts to flatten or go higher, it means we're failing at maintenance and we should think harder on solving the maintainability issues. 