time for calculated query answers needs to reflect actual data

When influxdb is used to select an individual result, the time associated with it is returned. e.g., if I do:

SELECT LAST(x) FROM table WHERE ... time > now() - 1d ....

then influxdb will give me output with the timestamp set to the last row from "table".

However if I do a query like this:

SELECT MAX(x) - MIN(x) FROM table WHERE ... time > now() - 1d ....

then influxdb uses the value of "now() - 1d" as the timestamp for the result "MAX(x) - MIN(x)". This is not useful as it cannot be aligned with any other timestamps in influxdb.

Ideally the timestamp for "MAX(x) - MIN(x)" would come from the actual timestamp of either the row for MAX(x) or MIN(x), not "now() - 1d"/"now()".

Start by tracing query execution for aggregate expressions and how result timestamps are assigned, comparing LAST(x) with MAX(x) - MIN(x). Confirm the intended timestamp semantics for rows contributing to calculated results, then add coverage showing that the result timestamp reflects actual data rather than the query-range boundary.

Written by the indexing model from the issue text.