Is it possible to add extra time value to timeFilter

Hi,
Is it possible toe add extra hours to my query $timeFilter

ex.: my current query is
SELECT mean("value") AS "some value name" FROM "cpu" WHERE time > now() - 8h GROUP BY time(1m) fill(previous)

When I change the timefilter I have only last 8h data, I want to add some extra hours to my time filter, something like this;

SELECT mean("value") AS "some value name" FROM "cpu" WHERE $timeFilter - 8h GROUP BY time(1m) fill(previous)

ex.: Last 7 days + 8h

regards,
Ganendran