How to parse a ZoneDateTime format? - postgresql

Question

I'm currently trying to parse a timestamp that looks like: 2020-08-03T11:37:42.529Z[UTC]
This timestamp was generated with Java's ZoneDateTime data type
What I have already tried was to parse it via:
SELECT '2020-08-03T11:37:42.529Z[UTC]'::timestamp with time zone;
But that would fail with an exception (unless I parse up until timezone Z).
Edit
For clarification, this is currently a String that is saved in a file, so this application has no direct interaction with Java.

Answer 1

As explained by Adrian, the below code (using at time zone) does not really work. The only other alternative I can think of is to replace the timezone abbreviation with a proper offset. To do that, a function is probably the easiest solution:
create or replace function replace_tz_abbrev(p_input text)
returns text
as
$$
declare
l_offset_hours text;
l_tz_abbrev text;
begin
l_tz_abbrev := substring(p_input from '\[([A-Z]+)\]');
select to_char(utc_offset, 'hh24:mi')
into l_offset_hours
from pg_catalog.pg_timezone_abbrevs
where abbrev = l_tz_abbrev;
return regexp_replace(p_input, '\[[A-Z]+\]', l_offset_hours);
end;
$$
language plpgsql;
This is a rough sketch, the function needs some error checking in case the abbreviation doesn't exist. Maybe checking pg_timezone_names as a fallback to deal with names like Europe/Berlin.
The result of replace_tz_abbrev() can be cast to a timestamptz (at least with the given example). This can either be done in the function itself (and changing it to returns timestamptz or when calling it.
The below is not correct
(I'll just leave it here for reference, so that the comments
One way I can think of is to extract the time zone from the string and use it together with the to_timestamp() function:
with data (input) as (
values ('2020-08-03T11:37:42.529Z[UTC]')
)
select to_timestamp(input, 'yyyy-mm-dd"T"hh24:mi:ss.ms') at time zone substring(input from '\[([A-Z]+)\]')
from data;

Answer 2

Assuming the timestamp string always ends with [timezone] then:
select regexp_replace('2020-08-03T11:37:42.529Z[UTC]', '\[[^\]]*\]', '')::timestamp with time zone;
regexp_replace
-----------------------------
08/03/2020 04:37:42.529 PDT
Where regexp_replace() replaces the [timezone] with an empty string and then you cast the 2020-08-03T11:37:42.529Z portion to a timestamp with time zone.

Answer 3

You can't directly convert this format to a timestamp with timezone. if you are allowed to manipulate the string to get the timedate separtly from the timezone you can do this:
select TIMESTAMP 'The date and time go here'AT TIME ZONE 'timezone name or abbreviation go here';
select TIMESTAMP '2020-08-03T11:37:42.529'AT TIME ZONE 'Africa/Dar_es_Salaam';
select TIMESTAMP '2020-08-03T11:37:42.529'AT TIME ZONE 'UTC';
The reason you have to split the string is that like you discovered a simple conversion does not work and the function TO_TIMESTAMP() that allows you to specify the format does not support specifiying in the format that the string will contain a timezone, only a time offset (example: -03 hours).
-- No way to include the timezone name in the format param.
-- if you insist to add 'TZ'or 'tz' you will get "ERROR: "TZ"/"tz"/"OF" format patterns are not supported in to_date"
select TO_TIMESTAMP('2020-08-03T11:37:42.529Z[UTC]','YYYY-MM-DD HH24:MI:SS.MS')
The difference between the a timezone and a time offset is also why you might not want to do what you are trying to do, postgres does not store timezones (or time offsets). Instead it converts to UTC, and later on reads the column as UTC if the column is 'timestamp without timezone', if the column is 'timestamp with timezone' it converts from UTC to what ever the current session is set to. if you care about the timezone you should store it in another column.