Date / Time

import java.time.*;
import java.time.format.DateTimeFormatter;
import java.time.temporal.ChronoUnit;
import java.time.temporal.TemporalAdjusters;

public class TimeDemo {
    public static void main(String[] args) {
        // 1) Instant — machine timestamp (UTC, no zone)
        Instant now = Instant.now();
        System.out.println(now);                                   // 2026-06-18T03:21:00.123Z
        System.out.println(now.toEpochMilli());

        // 2) LocalDate / LocalTime / LocalDateTime — no zone
        LocalDate today    = LocalDate.now();
        LocalDate eom      = today.with(TemporalAdjusters.lastDayOfMonth());
        LocalDate next     = today.plusMonths(1);
        LocalTime nine     = LocalTime.of(9, 30);
        LocalDateTime stamp= LocalDateTime.of(today, nine);
        System.out.println("eom=" + eom + " next=" + next + " stamp=" + stamp);

        // 3) ZonedDateTime — wall clock at a specific zone
        ZoneId syd = ZoneId.of("Australia/Sydney");
        ZoneId hk  = ZoneId.of("Asia/Hong_Kong");
        ZonedDateTime sydMeeting = ZonedDateTime.of(stamp, syd);
        ZonedDateTime hkMeeting  = sydMeeting.withZoneSameInstant(hk);
        System.out.println("Sydney meeting @ HK time: " + hkMeeting);

        // 4) Parse + format — ISO-8601 and custom
        ZonedDateTime parsed = ZonedDateTime.parse("2026-06-18T09:30:00+10:00[Australia/Sydney]");
        String formatted = parsed.format(
            DateTimeFormatter.ofPattern("EEE d MMM yyyy h:mm a z"));
        System.out.println(formatted);

        // 5) Duration vs Period
        Duration runtime = Duration.between(now.minusSeconds(125), now);
        System.out.println("runtime: " + runtime.toMinutesPart() + "m " + runtime.toSecondsPart() + "s");

        Period age = Period.between(LocalDate.of(2020, 1, 1), today);
        System.out.println("age: " + age.getYears() + " years " + age.getMonths() + " months");

        // 6) Arithmetic without surprises
        ZonedDateTime in7 = sydMeeting.plus(7, ChronoUnit.DAYS);
        boolean weekend   = in7.getDayOfWeek() == DayOfWeek.SATURDAY
                          || in7.getDayOfWeek() == DayOfWeek.SUNDAY;

        // 7) Daylight saving transitions — same wall clock, different offsets
        ZonedDateTime preDst  = ZonedDateTime.parse("2026-04-05T01:30:00+11:00[Australia/Sydney]");
        ZonedDateTime postDst = preDst.plus(1, ChronoUnit.HOURS);
        System.out.println("before DST: " + preDst);
        System.out.println("after DST:  " + postDst);

        // 8) Clock — inject for tests so "now" is deterministic
        Clock fixed = Clock.fixed(Instant.parse("2026-06-18T00:00:00Z"), ZoneOffset.UTC);
        LocalDate dt = LocalDate.now(fixed);                       // always 2026-06-18

        // 9) Convert to/from legacy Date when an old API forces you to
        java.util.Date legacy = java.util.Date.from(now);
        Instant back = legacy.toInstant();

        // 10) Decision matrix
        // - Machine timestamps (logs, audit, sort by event time)   -> Instant
        // - Calendar date with no time                              -> LocalDate
        // - Wall clock at a known zone (UI display, scheduling)     -> ZonedDateTime
        // - Elapsed measurement                                      -> Duration
        // - Calendar offset (1 month, 3 years)                       -> Period
    }
}

import java.time.*;
LocalDate today = LocalDate.now();
LocalDateTime stamp = LocalDateTime.now();
Duration d = Duration.between(stamp.minusHours(2), stamp);