All 15 Java Date & Time API (java.time) questions
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How would you use TemporalAdjusters to compute dates like 'the next Monday' or 'the last day of the month' relative to a given LocalDate? How would you calculate a person's age or the number of days until a recurring annual event (like a birthday) using ChronoUnit, and what's a common pitfall with leap-year birthdays (February 29)? What is the significance of storing date-time values in UTC in a database or when transmitting between systems, rather than storing them in a specific local timezone? How would you determine whether a given year is a leap year, and calculate the number of days in a specific month, using the java.time API's built-in support rather than hand-writing this classic calendrical logic? How would you correctly handle daylight saving time (DST) transitions when performing date-time arithmetic with ZonedDateTime, and what specific pitfalls exist around the 'spring forward' and 'fall back' transitions? How would you convert between the legacy java.util.Date/Calendar classes and the modern java.time API when working with older libraries or APIs that still require the legacy types? How would you implement a custom TemporalAdjuster to calculate business-specific date logic (like 'the next business day, skipping weekends and holidays')? How would you correctly serialize and deserialize java.time types (like LocalDateTime or ZonedDateTime) to/from JSON using a library like Jackson, and what common configuration pitfalls arise? How would you implement a recurring event scheduling system that correctly handles both fixed-instant recurrence (every exactly 24 hours) and calendar-based recurrence (every day at 9 AM local time), given these have meaningfully different behavior across DST transitions?
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How does the Instant class represent a point in time, and how does it differ fundamentally from ZonedDateTime in terms of what it actually represents and how it should be used?

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Answer

Instant represents a single, unambiguous point on the timeline measured as elapsed time since the epoch (1970-01-01T00:00:00Z), stored internally with nanosecond precision, entirely independent of any timezone or human-readable calendar representation -- it's the appropriate choice for timestamping events (like a log entry's creation time, or a database record's last-modified timestamp) where you need an unambiguous, machine-comparable moment in time without caring about how a human would read it in their local calendar; ZonedDateTime, by contrast, is specifically for representing and DISPLAYING a date-time in a particular timezone's local calendar convention, converting an Instant's underlying universal moment into a human-meaningful year/month/day/hour/minute representation for a specific place.
Instant now = Instant.now();  // pure timestamp, e.g. 2024-06-01T18:30:00Z, no calendar/timezone concept
System.out.println(now);

// Convert to a human-readable local representation for a SPECIFIC timezone when needed for display
ZonedDateTime nyTime = now.atZone(ZoneId.of("America/New_York"));
ZonedDateTime tokyoTime = now.atZone(ZoneId.of("Asia/Tokyo"));
// Both represent the EXACT SAME underlying instant, just displayed differently for each timezone
Real-world example A distributed logging system timestamps every log entry using Instant (a pure, unambiguous, universally comparable moment), only converting to a ZonedDateTime for a specific timezone at the point of DISPLAYING logs to a user in their local dashboard, correctly separating the underlying data model (Instant) from its presentation (ZonedDateTime).

Common follow-ups: Why is it generally considered bad practice to store Instant values converted to a specific ZonedDateTime in a database, rather than storing the Instant/UTC value directly?;How does Instant's nanosecond precision compare to the precision typically available from java.util.Date's millisecond-based design?

Logging in Java (java.util.logging SLF4J Log4j);Concurrency & Threads
All 15 Java Date & Time API (java.time) questions