All 15 Java Date & Time API (java.time) questions
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Why was the modern java.time API (introduced in Java 8) created to replace the legacy Date and Calendar classes, and what specific problems did the old classes have? How would you get the current date and time, and how does specifying an explicit ZoneId (versus relying on the system default) affect the result of LocalDate.now() or LocalDateTime.now()? How would you calculate the difference between two dates or times using Period and Duration, and what's the distinction between these two classes? How do you parse and format date/time values using DateTimeFormatter, and how does this replace the older, notoriously thread-unsafe SimpleDateFormat class? 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 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? 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?
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What is the difference between LocalDate, LocalTime, LocalDateTime, and ZonedDateTime, and when would you use each?

Intermediate

Answer

LocalDate represents a date without any time-of-day or timezone component (like a birthday, January 15); LocalTime represents a time-of-day without any date or timezone (like 'every day at 9:00 AM'); LocalDateTime combines both a date and time but STILL has no timezone information attached, representing a date-time as observed in some unspecified, implicit location; ZonedDateTime adds explicit timezone information to a date-time, correctly representing a genuine, unambiguous point in time across the globe -- choosing the right one depends on whether your data genuinely needs a timezone (anything crossing time zones, like scheduling a meeting across offices, needs ZonedDateTime; local, timezone-independent concepts like a birthday or a recurring daily local event fit LocalDate/LocalTime/LocalDateTime).
LocalDate birthday = LocalDate.of(1990, 5, 15);              // just a date, no time or zone
LocalTime lunchTime = LocalTime.of(12, 30);                    // just a time, no date or zone
LocalDateTime meeting = LocalDateTime.of(2024, 6, 1, 14, 0);    // date+time, but no zone -- ambiguous globally
ZonedDateTime flight = ZonedDateTime.of(meeting, ZoneId.of("America/New_York"));  // fully unambiguous
Real-world example A flight booking system stores departure times as ZonedDateTime (since a flight's exact moment in universal time genuinely matters and must be unambiguous across timezones), while a user's stored birthday uses LocalDate (since a birthday is a timezone-independent, purely calendrical concept that shouldn't shift depending on where the user happens to be).

Common follow-ups: Why is LocalDateTime specifically dangerous to use for representing a genuine, globally-unambiguous instant in time?;How does Instant differ from all four of these types, and when would you use it instead?

Java Networking & HTTP Client;JVM JRE & Memory
All 15 Java Date & Time API (java.time) questions