Date and Time with java.time

java.util.Date mixed instants with human dates and was mutable to boot. The java.time package separates these concepts into clear, immutable types.

Choose the right type

Use Instant for machine timestamps, LocalDate for calendar dates, and ZonedDateTime when a time zone matters.

Instant now = Instant.now();                       // UTC timestamp
LocalDate today = LocalDate.now();                 // no time, no zone
ZonedDateTime meeting = ZonedDateTime.of(
    2026, 5, 1, 9, 30, 0, 0, ZoneId.of("America/New_York"));

Do arithmetic safely

Durations and periods make offsets explicit and DST-aware.

Instant deadline = now.plus(Duration.ofHours(48));
LocalDate nextMonth = today.plus(Period.ofMonths(1));

Takeaways

• Store and transmit timestamps as UTC Instant; convert to zones only for display.
• Never use Date or Calendar in new code.
• Use DateTimeFormatter (immutable, thread-safe) instead of SimpleDateFormat.

Persist times as UTC in the database and apply the user's zone at the edge, so comparisons and sorting stay correct.