Why postgres saves date in different ways?

I have Spring Boot app with PostgreSQL. When I save a date like 2020-07-28 в 17:30:00, in PostgreSQL it looks 2020-07-28 в 06:30:00. Sometimes the difference is 12 hours sometimes 11. In front-end it comes in right way, but when I export it in cvs wrong date comes out. Why it happens and how it fix?
These are my variables:

    @NotNull
    @DateTimeFormat(pattern = "dd/mm/yyyy")
    private Date startDate;

    @DateTimeFormat(pattern = "dd/mm/yyyy")
    private Date endDate;

The java.util.Date is pretty annoying to be honest. If you want to play with it, in database or in front you should better use LocalDateTime as said. However if you want to continue using java.util.Date, I have found a way that worked for me but is pretty much just bypassing the problem. If you want you can try using it like that :

 Date startDate;
 SimpleDateFormat formater = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
 System.Out.println(formater.format(startDate));

I used this to insert in DB and in front-end and worked pretty well for me

Avoid Date

Never use the java.util.Date or java.sql.Date classes. Along with Calendar and SimpleDateFormat, these are now legacy, supplanted years ago by the modern java.time classes.

java.time classes with JDBC 4.2+

If you are trying to record a moment, a specific point on the timeline, use:

• In Postgres, a column of type TIMESTAMP WITH TIME ZONE. Notice the WITH rather than WITHOUT (which cannot represent a moment).
• In Java, Instant, OffsetDateTime, or ZonedDateTime.

JDBC 4.2 and later lets us exchange java.time objects with the database. But oddly, JDBC 4.2 requires support only for OffsetDateTime but not the two more commonly used types. Fortunately, converting is easy.

LocalDate ld = LocalDate.of(  2020 , Month.JULY , 28 ) ;
LocalTime lt = LocalTime.of( 17 , 30 ) ;
ZoneId z = ZoneId.of( "Asia/Tokyo" ) ;
ZonedDateTime zdt = ZonedDateTime.of( ld , lt , z ) ;

You may be able to pass that to your particular JDBC driver.

myPreparedStatement.setObject( … , zdt ) ;

If not, convert to OffsetDateTime.

myPreparedStatement.setObject( … , zdt.toOffsetDateTime() ) ;

Retrieval.

OffsetDateTime odt = myResultSet.getObject( … , OffsetDateTime.class ) ;

Adjust to your desired time zone.

ZoneId z = ZoneId.of( "Asia/Tokyo" ) ;
ZonedDateTime zdt = odt.atZoneSameInstant​( z ) ;

Present to the user, as text, localized.

Locale locale = Locale.CANADA_FRENCH ;
DateTimeFormatter f = DateTimeFormatter.ofLocalizedDateTime( FormatStyle.FULL ).withLocale( locale ) ;
String output = zdt.format( f ) ;

See the same moment in UTC. A Instant object is always in UTC, by definition.

Instant instant = zdt.toInstant() ;

Using the code seen here will eliminate your time zone surprises.

All this has been covered many many many times already on Stack Overflow. So I was brief. Search to learn more.