I wrote a simple Java entity with two Instant fields:

@Entity
@Table(name = "tb_foo")
public class Foo {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(columnDefinition = "TIMESTAMP WITHOUT TIME ZONE")
    private Instant date1;
    
    @Column(columnDefinition = "TIMESTAMP WITHOUT TIME ZONE")
    private Instant date2;
    ...

Then I wrote a simple import.sql in the project's resources root folder to test if UTC/Local is working:

INSERT INTO tb_foo (date1, date2) VALUES ('2020-07-22T20:50:01.12345Z', NOW());

As my local time zone is GMT-3, that 20:50:01 above should appear on client as 17:50:01, and that NOW() should appear equivalent to my local time (around 0:25 at the moment I tested). I tested it in H2 database and it worked perfectly:

This is the DDL & INSERT auto generated by JPA to H2:

create table tb_foo (
    id bigint generated by default as identity,
    date1 TIMESTAMP WITHOUT TIME ZONE,
    date2 TIMESTAMP WITHOUT TIME ZONE,
    primary key (id)
)
INSERT INTO tb_foo (date1, date2) VALUES ('2020-07-22T20:50:01.12345Z', NOW()) 

However, when I tested the same Java and SQL codes on Postgres (12.x), it didn't work (20h UTC showed as 20 instead of 17):

This is the DDL & INSERT auto generated by JPA to Postgresql:

create table tb_foo (
    id int8 generated by default as identity,
    date1 TIMESTAMP WITHOUT TIME ZONE,
    date2 TIMESTAMP WITHOUT TIME ZONE,
    primary key (id)
)
INSERT INTO tb_foo (date1, date2) VALUES ('2020-07-22T20:50:01.12345Z', NOW())

What did I miss?

Update:

I've made another test: I wrote another database insertion through Java (not SQL INSERT):

Foo foo = new Foo(null, Instant.parse("2020-07-23T20:50:01Z"), Instant.now());
fooRepository.save(foo);

And the literal UTC timestamp 20:50 was properly inserted into Postgresql this time! It appeared as 17:50 on pgadmin client! So maybe the right question is:

How to properly specify a literal UTC timestamp to Postgresql in a SQL INSERT call to store it in a timestamp WITHOUT time zone?

I've tried it in two ways:

INSERT INTO tb_foo (date1, date2) VALUES ('2020-07-22T20:50:01.12345Z', NOW());

INSERT INTO tb_foo (date1, date2) VALUES ('2020-07-22 20:50:01 +00', NOW());

And both didn't work (they both show 20:50 instead of 17:50 on the pgadmin client in a GMT-03 system).