drop table date_test;

CREATE TABLE date_test (
date_tz TIMESTAMP WITH TIME ZONE
,date_wtz TIMESTAMP
);


insert into date_test(date_tz) values('0018-12-06 17:16:58.829');
insert into date_test(date_tz) values('0218-12-06');

select * 
from date_test;

update date_test
set date_wtz = date_tz;

select * from date_test;

0018-12-06 17:16:58.829
0218-12-06 17:11:58.829

=> \d+ source_table
                                   Table "public.source_table"
      Column      |           Type           |       Modifiers        | Storage  | Stats target | Description
------------------+--------------------------+------------------------+----------+--------------+-------------
 fld1             | integer                  | not null               | plain    |              |
 time_field       | timestamp with time zone | not null               | plain    |              |
 fld2             | integer                  | default 0              | plain    |              |
 fld3             | text                     | default ''::text       | extended |              |
 fld4             | smallint                 |                        | plain    |              |
 fld5             | timestamp with time zone | not null default now() | plain    |              |
 fld6             | bigint                   |                        | plain    |              |
Indexes:
    "abc_source_table2" btree (time_field, fld1)
Triggers:
    insert_source_table_redirect_trigger BEFORE INSERT ON source_table FOR EACH ROW EXECUTE PROCEDURE insert_source_table_redirect_function()
Child tables: source_table_20181224,
              source_table_20181225,
              source_table_20181226,
              source_table_20181227,
              source_table_20181228
Has OIDs: no

select 
	 EXTRACT(YEAR  FROM time_field) as year
	,EXTRACT(MONTH FROM time_field) as month
	,EXTRACT(DAY FROM   time_field) as day
	,count(*)
from source_table
group by 
	 EXTRACT(YEAR FROM  time_field)
	,EXTRACT(MONTH FROM time_field)
	,EXTRACT(DAY FROM   time_field);

 year | month | day | count
------+-------+-----+-------
 2018 |    12 |  25 | 51552
 2018 |    12 |  24 | 51573
 2018 |    12 |  26 | 51589
 2018 |    12 |  28 | 20605
 2018 |    12 |  27 | 51433
(5 rows)