Group by Time Question

I have a query that returns the buckets I want when I request data up to now()
select * from measurement where time >= ‘2020-03-15’ and time <= now() group by time(7m, now)

How can I get the same behavior when I want data up to a date? as this is not valid:
select * from measurement where time >= ‘2020-03-15’ and time <= ‘2020-03-20’ group by time(7m, ‘2020-03-15’)
And using now() in the group clause doesn’t do the behavior I want in this case.

The behavior I want is to not use the preset buckets and to have the buckets on the time where I ask them.

Thanks!