Re: Issue with date/timezone conversion function

On Tue, Apr 9, 2024 at 10:33 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Lok P <loknath(dot)73(at)gmail(dot)com> writes:
> > These tables are INSERT only tables and the data in the create_timestamp
> > column is populated using the now() function from the application, which
> > means it will always be incremental, and the historical day transaction
> > count is going to be the same. However surprisingly the counts are
> changing
> > each day when the user fetches the result using the below query. So my
> > question was , if there is any issue with the way we are fetching the
> data
> > and it's making some date/time shift which is why the transaction count
> > looks to be changing even for the past days data?
>
> Well, your cutoff time "CURRENT_TIMESTAMP - INTERVAL '10 day'" is
> constantly moving, so that'd account for shifts in what's perceived
> to belong to the oldest day. Maybe you want "CURRENT_DATE - 10"
> instead?
>
> > And also somehow this
> > conversion function "DATE_TRUNC('hour', create_timestamp AT TIME ZONE
> > 'EST')" is showing time in CST but not EST, why so?
>
> 'EST' is going to rotate to UTC-5, but that's probably not what
> you want in the summer. I'd suggest AT TIME ZONE 'America/New_York'
> or the like. See
>
>
> https://www.postgresql.org/docs/current/datatype-datetime.html#DATATYPE-TIMEZONES
>
> regards, tom lane
>

Thank you so much. You are correct. The AT TIME ZONE 'America/New_York' is
giving correct EST time conversion.

But I think regarding why it looks to be shifting i.e. the same time
duration appears to be holding a different count of transactions while the
base table is not getting updated/inserted/deleted for its historical
create_timestamps, I suspect the below conversion part.

The task is to count each ~15minutes duration transaction and publish in
ordered fashion i.e. something as below, but the way it's been written
seems wrong. It's an existing script. It first gets the date component with
truncated hour and then adds the time component to it to make it ~15minutes
interval. Can it be written in some simple way?

9-apr-2024 14:00 12340
9-apr-2024 14:15 12312
9-apr-2024 14:30 12323
9-apr-2024 14:45 12304

*DATE_TRUNC('hour', create_timestamp AT TIME ZONE '*America/New_York'*) +*
*(EXTRACT(MINUTE FROM create_timestamp AT TIME ZONE '*America/New_York*') /
15 * 15) * INTERVAL '15 minute'*