timezone aware date_trunc function
While the marked answer might be correct for the OP's weird circumstances it is more likely incorrect for others. You need to convert the timestamp returned by date_trunc to the proper timezone.
select
date_trunc(
'day',
some_timestamp at time zone users_timezone
) at time zone users_timezone as the_date;
The important thing to understand is date_trunc
returns a timestamp
with no timezone attached to it. You need to convert the timestamp to the proper timezone because the database client or whatever downstream might have a different timezone.
You need to specify at which time zone you want it to show
select
date_trunc(
'day',
timestamp with time zone '2001-01-1 00:00:00+0100' at time zone '-02'
) as the_date;
the_date
---------------------
2001-01-01 00:00:00
AT TIME ZONE
@Adam's answer is definitely more helpful. Although I think we can improve on it again because if we're truncating a Timestamp to a single day (or week/month/etc), then we want to make sure that we're dealing with a Date object, not a Timestamp. Otherwise we may give other pieces of code the impression that something just actually happened to occur at midnight (or potentially some other misleading time of day).
So I would use:
SELECT date_trunc('day', some_timestamp AT TIME ZONE users_timezone)::date AS the_date;
which casts the result to a Date, rather than Timestamp.
The result will be something like:
the_date
------------
2019-09-14
instead of the more misleading result of:
the_date
---------------------
2019-09-14 00:00:00