I’m not at all sure that MIN/MAX aggregations are implemented for DateTime Group types. Not every function makes sense for every possible type e.g. what should AVG or SUM do with dates?
The docs do say that MIN/MAX/AVG are for “decimal type” (Number)
EDIT - so, how to do it instead? Using Group functions is a neat idea, but you’d have to do it with a Group of Number Items holding the “timestamp” as an epoch integer.
Easy enough to populate those from your timestamping rule.
Or abandon the group, and have your rule populate a dedicated “oldest” Item. The rule would have to retrieve the “old” individual value before updating the timestamp, and compare with current “oldest”.
Hmm, then you’d still need to search for the next-oldest. That’s an interesting little task … I can feel a sorted list coming on somewhere.