LocalDateTime

// ── LocalDateTime — every field of LocalDate AND LocalTime combined ──
LocalDateTime meeting = LocalDateTime.of(2024, 6, 15, 14, 30);
// year, month, day, hour, minute all present — but STILL no zone/offset anywhere

System.out.println(meeting);   // 2024-06-15T14:30

// ── The SAME LocalDateTime is ambiguous about which real-world instant it is
LocalDateTime ambiguous = LocalDateTime.of(2024, 6, 15, 14, 30);
// Is this 2:30 PM in Tokyo? New York? UTC? The object itself cannot say —
// it was never given that information, and has no field to store it in

// ── Correct use case — a single office's own local wall-clock concept ─
LocalDateTime localAppointment = LocalDateTime.of(2024, 6, 15, 9, 0);
// fine AS LONG AS everyone reading it shares the same understood local context

// ── Building from raw fields, or from existing LocalDate + LocalTime ──
LocalDateTime fromFields = LocalDateTime.of(2024, 6, 15, 14, 30, 0);

LocalDate date = LocalDate.of(2024, 6, 15);
LocalTime time = LocalTime.of(14, 30);
LocalDateTime fromParts = LocalDateTime.of(date, time);   // combine two existing pieces

// ── atTime / atDate — combine from EITHER side, same result ───────────
LocalDateTime viaAtTime = date.atTime(time);
LocalDateTime viaAtDate = time.atDate(date);
System.out.println(viaAtTime.equals(viaAtDate));   // true — same combined value

// ── Decomposing — pull just one half back out when that's all you need
LocalDate justDate = fromParts.toLocalDate();   // 2024-06-15 — time discarded
LocalTime justTime = fromParts.toLocalTime();   // 14:30 — date discarded

LocalDateTime wallClockTime = LocalDateTime.of(2024, 6, 15, 14, 30);

// ── atZone — ATTACH an interpretation, fields themselves stay UNCHANGED
ZonedDateTime asTokyo = wallClockTime.atZone(ZoneId.of("Asia/Tokyo"));
System.out.println(asTokyo);   // 2024-06-15T14:30+09:00[Asia/Tokyo]
// Note: still "14:30" — atZone did NOT convert the time, only declared its zone

// ── From there, .toInstant() gives a true, zone-independent universal moment
Instant instant = asTokyo.toInstant();
System.out.println(instant);   // 2024-06-15T05:30:00Z — UTC equivalent of 14:30 Tokyo time

// ── Same LocalDateTime, DIFFERENT declared zone → DIFFERENT real instant
ZonedDateTime asNewYork = wallClockTime.atZone(ZoneId.of("America/New_York"));
System.out.println(asNewYork.toInstant());   // a completely different universal moment
// even though the wall-clock numbers "14:30" were identical going in