I have found a mistake in OPC UA Part 13 - Aggregates Specification (v1.03).
At least chapters '5.4.3.10 Minimum' and '5.4.3.11 Maximum' have contradictory
definition of an Aggregate timestamp:
in the first paragraph: 'returns that value with the timestamp at which that value occurs';
but the third paragraph states: 'The timestamp of the Aggregate will always be the start of the interval for every ProcessingInterval'.

Attached screenshot of the specification shows exact places in chapter '5.4.3.10 Minimum'

So, it is unclear which timestamp to use for these Aggregates: timestamp of minimum/maximum value or start time of the interval.