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()? What is the difference between LocalDate, LocalTime, LocalDateTime, and ZonedDateTime, and when would you use each? 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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How would you calculate the difference between two dates or times using Period and Duration, and what's the distinction between these two classes?

Intermediate

Answer

Period represents a date-based amount of time in human calendar terms (years, months, days), appropriate for calculating differences between LocalDate values where calendar irregularities matter (like a month having a variable number of days); Duration represents a time-based amount measured in exact seconds/nanoseconds, appropriate for calculating differences between time-based or instant-based values (LocalTime, Instant) where you want a precise, fixed-length elapsed time rather than a calendar-relative one -- using the wrong one for a given scenario (like computing a Duration between two LocalDate values) either won't compile or won't give calendar-meaningful results.
LocalDate start = LocalDate.of(2024, 1, 15);
LocalDate end = LocalDate.of(2024, 6, 20);
Period period = Period.between(start, end);
System.out.println(period.getMonths() + " months, " + period.getDays() + " days");  // calendar-aware difference

Instant startTime = Instant.now();
Instant endTime = startTime.plusSeconds(3661);
Duration duration = Duration.between(startTime, endTime);
System.out.println(duration.toHours() + "h " + duration.toMinutesPart() + "m");  // exact elapsed time
Real-world example An age-calculation feature uses Period.between() to correctly compute a person's age in calendar-meaningful years and months (correctly accounting for varying month lengths and leap years), while a separate performance-monitoring feature uses Duration.between() on two Instant timestamps to measure precise elapsed processing time in milliseconds for a request.

Common follow-ups: What happens if you try to compute a Period between two Instant values, or a Duration between two LocalDate values -- does it compile?;How does Period.between() handle the calculation when the day-of-month values don't align evenly (like from January 31 to March 1)?

Diagnostics & Performance;Java Fundamentals: Syntax Data Types & Operators
All 15 Java Date & Time API (java.time) questions