Date

public static void main(String[] args) {
    final Date date = new Date();
    final GregorianCalendar calendar = new GregorianCalendar();
    final SimpleDateFormat dateFormat = new SimpleDateFormat();

    //Instant는 기계용 데이터임. 시간을 재거나 메소드 실행시간을 비교할때 사용.
    final Instant instant = Instant.now();
    System.out.println("instant = " + instant); // UTC, GMT 기준
    System.out.println("instant = " + instant.atZone(ZoneId.of("UTC"))); // UTC로 표현. UTC와 GMT시간은 같다.

    //현재 내가 있는곳과 시간을 나타내줌.
    final ZoneId zone = ZoneId.systemDefault();
    System.out.println("zone = " + zone);
    final ZonedDateTime zonedDateTime = instant.atZone(zone);
    System.out.println("zonedDateTime = " + zonedDateTime);
    //출력결과 = zonedDateTime = 2022-12-21T18:11:49.123541+09:00[Asia/Seoul]

    LocalDateTime now = LocalDateTime.now();
    LocalDateTime birthDay = LocalDateTime.of(1982, Month.JULY, 15, 0, 0, 0);

    LocalDate today = LocalDate.now();
    LocalDate thisYearBirthday = LocalDate.of(2023, Month.JUNE, 12);
    ZonedDateTime.now(ZoneId.of("Asia/Seoul"));

    //Period는 날짜와 날짜끼리 얼마나 차이가 있는지 비교함. (휴먼용)
    final Period period = Period.between(today, thisYearBirthday);
    final Period until = today.until(thisYearBirthday);
    System.out.println("period = " + period.get(ChronoUnit.DAYS));
    System.out.println("until = " + until.get(ChronoUnit.DAYS));

    //Duration도 날짜와 날짜끼리 얼마나 차이가 있는지 비교함. (머신용)
    Instant now1 = Instant.now();
    Instant plus = now1.plus(10, ChronoUnit.SECONDS);
    final Duration between = Duration.between(now1, plus);
    System.out.println("between = " + between.getSeconds());

    final LocalDateTime now2 = LocalDateTime.now();
    final DateTimeFormatter MMddyyyy = DateTimeFormatter.ofPattern("MM/dd/yyyy");
    System.out.println("now2.format(MMddyyyy) = " + now2.format(MMddyyyy));

    final LocalDate parse = LocalDate.parse("07/15/1982", MMddyyyy);
    System.out.println("parse = " + parse);

}