Postgresql extracting 'epoch' from timestamp cuts off last date in date range - postgresql

Question

My table has the column event_ts with column type numeric.
Here is my query:
select
min(to_timestamp(event_ts)), max(to_timestamp(event_ts))
from
table1
where
event_ts >= extract('epoch' from '2021-07-01'::timestamp) and
event_ts <= extract('epoch' from '2021-07-31'::timestamp)
However, the results are
min: 2021-06-30 20:00:00.000 -0400
max: 2021-07-30 20:00:00.000 -0400
I would think the where clause would include data from 2021-07-01 to 2021-07-31.
There is data for July 31st, 2021.
Why does this query start at 2021-06-30 and end 2021-07-30?

Answer 1

show timezone;
TimeZone
------------
US/Pacific
select extract('epoch' from '2021-07-01'::timestamp);
extract
-------------------
1625097600.000000
select to_timestamp(1625097600);;
to_timestamp
-------------------------
06/30/2021 17:00:00 PDT
select extract('epoch' from '2021-07-01'::timestamptz);
extract
-------------------
1625122800.000000
(1 row)
test(5432)=# select to_timestamp(1625122800);
to_timestamp
-------------------------
07/01/2021 00:00:00 PDT
So by using timestamp you are creating a local time offset by the timezone offset. Using timestamptz will return a timestamp at 0:00:00.
This is because from here:
https://www.postgresql.org/docs/current/functions-datetime.html#FUNCTIONS-DATETIME-EXTRACT
epoch
For timestamp with time zone values, the number of seconds since 1970-01-01 00:00:00 UTC (negative for timestamps before that); for date and timestamp values, the nominal number of seconds since 1970-01-01 00:00:00, without regard to timezone or daylight-savings rules; for interval values, the total number of seconds in the interval
Epoch is based on UTC timezone.
Not sure why you are using epoch anyway?
Why not?:
...
where
event_ts between '2021-07-01'::timestamptz and '2021-07-31'::timestamptz