Date and Time

✅ LocalDateTime

• 👍🏻 get time of my local area

• LocalDate: 2023-11-21
• LocalTime: 00:20:30.234

• LocalDateTime: LocalDate + LocalTime, 2023-11-21T00:20:30.234

• now()

LocalDate now = LocalDate.now(); //2025-05-01
LocalTime now = LocalTime.now(); //17:55:06.305827

LocalDateTime now = LocalDateTime.now(); //2025-05-01T17:57:05.637786
LocalDate date = now.toLocalDate(); //get only date from LocalDateTime
LocalTime time = now.toLocalTime(); //get only time from from LocalDateTime
LocalDateTime addDateTime = LocalDateTime.of(date, time); //날짜, 시간 합치기

• isBefore()
• isAfter()
• isEquals()

✅ ZonedDateTime

• UTC, summertime에 대한 정보 포함한 시간
• for developing international time
• 👍🏻 apply both UTC, summertime

ZoneId.getAvailableZoneIds() // 내가 시간 얻을 수 있는 지역 얻기
ZoneId zoneId = ZoneId.systemDefault(); //내 시스템이 어디 지역 시간 쓰는지

• ✔️ Get other country times

ZonedDateTime seoul = ZonedDateTime.of(LocalDate.of(2024, 1, 1), LocalTime.of(9, 0, 0), ZoneId.of("Asia/Seoul"));
ZonedDateTime london = seoul.withZoneSameInstant(ZoneId.of("Europe/London"));
ZonedDateTime newyork = seoul.withZoneSameInstant(ZoneId.of("America/New_York"));

✅ OffsetDateTime

• UTC에 대한 정보 포함한 시간
• 👍🏻 apply UTC, do not apply summertime

✅ Instant

• how much time passed since 1970-01-01 00:00:00
• show how much seconds passed
• 👍🏻 used when the time has to same all over the world

Instant now = Instant.now();
Instant epochSecond = Instant.ofEpochSecond(0);

✅ Period and Duration

• amount of time

• ✔️ Period
• amount of time in year, month, days

Period period = Period.ofDays(10); //10days


LocalDate localDate = LocalDate.of(2020, 3, 3);
LocalDate plus = localDate.plus(period); //3 + 10 = 13일


//기간의 차이 구하기
LocalDate startDate = LocalDate.of(2020, 3, 3);
LocalDate endDate = LocalDate.of(2025, 5, 1);

Period between = Period.between(startDate, endDate);
between.getYears(); //5년
between.getMonths(); //1개월
between.getDays(); //28일

• ✔️ Duration
• amount of time in hour, minutes, seconds

✅ Datetime Interface

• LocalDateTime, ZonedDateTime, OffsetDateTime extends interface Temporal and TemporalAccessor
• ✔️ interface TemporalAccessor: read date, time

• ✔️ interface Temporal: modify date, time

• Period and Duration extends interface TemporalAmount

• ✔️ interface TemporalAmount: amount of time

• ChronoUnit extends interface TemporalUnit
• ✔️ interface TemporalUnit: 시간 단위 제공

• minutes, day, years, decades…

• ChronoField extends interface TemporalField
• ✔️ interface TemporalField: 날짜와 시간의 특정 부분을 나타냄
  • 예를 들어 8월 16일일 때, MONTH_OF_YEAR: 8, DAY_OF_YEAR: 16

ChronoField.MONTH_OF_YEAR.range(); //1-12
ChronoField.DAY_OF_YEAR.range() //1-365/366

✅ Get and modify Date, time

• ✔️ Get date, time

LocalDateTime dt = LocalDateTime.of(2020, 1, 2, 3, 40, 50);

dt.get(ChronoField.YEAR); //2020
dt.getYear(); //2020

• ✔️ Add time
• same result, in three ways

LocalDateTime dt = LocalDateTime.of(2020, 1, 2, 3, 40, 50);

// 1️⃣ ChronoUnit
LocalDateTime years1 = dt.plus(10, ChronoUnit.YEARS); //2030
// 2️⃣ plusYears(simplified method of ChronoUnit)
LocalDateTime years2 = dt.plusYears(10); //2030

3️⃣ Period 사용
Period period = Period.ofYears(10);
LocalDateTime years3 = dt.plus(period); //2030

• ✔️ Check if I can get this value from this class

LocalDate now = LocalDate.now();
now.isSupported(ChronoField.SECOND_OF_DAY); //can I get SECOND_OF_DAY from LocalDate

• ✔️ Modify time
• with()

LocalDateTime dt = LocalDateTime.of(2020, 1, 2, 3, 40, 50);
LocalDateTime changedDt1 = dt.with(ChronoField.YEAR, 2030); //change year to 2030

LocalDateTime changedDt2 = dt.withYear(2030);

//temporal adjuster
//change to next week
LocalDateTime nextFriday = dt.with(TemporalAdjusters.next(DayOfWeek.FRIDAY));
//last sunday of this month
LocalDateTime sameMonthLastSunday = dt.with(TemporalAdjusters.lastInMonth(DayOfWeek.SUNDAY));

✅ Date ➡️ String

• ✔️ Date ➡️ String

LocalDateTime dateTime = LocalDateTime.of(2020, 1, 2, 3, 40, 59);

DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy년 MM월 dd일 HH:mm:ss");
String formattedDateTime = dateTime.format(formatter); //2020년 01월 02일 03:40:59

• ✔️ String ➡️ Date

String input = "2020년 09월 22일 12:34:32";
LocalDateTime parsedDateTime = LocalDateTime.parse(input, formatter); //2020-09-22T12:34:32

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