Timestamp derived data does not cater for source data going stale

[marcobergman]

When one of multiple sources for a derived value goes stale (does not receive updates anymore), but the other sources still update, the derived data is still calculated with fresh timestamps, although it is based on old sensor values, so to speak.

This would become an issue when data displays rely on timestamps to decide whether to give an indication of stale data, e.g. crossed out or empty display.

IMHO the timestamp of a derived value should be the minimum of the timestamps of all source values.