Java Date and Time API: java.time and the Current Date - Quiz

The Java Date and Time API is the java.time package (JSR-310), the standard library for dates and times added in Java 8 to replace java.util.Date and java.util.Calendar.

Main classes:

• LocalDate — a date without a time (2026-10-17);
• LocalTime — a time of day (14:35:07);
• LocalDateTime — a date and time without a time zone;
• ZonedDateTime — a date and time in a specific zone (America/New_York);
• Instant — a moment on the UTC time-line;
• Period and Duration — amounts of time in years/months/days and in seconds/nanoseconds.

All of them are immutable and thread-safe. The point-in-time classes implement java.time.temporal.Temporal, the amount classes implement java.time.temporal.TemporalAmount.

With the static now() method of the right java.time class:

• LocalDate.now() — the date only: 2026-10-17;
• LocalTime.now() — the time only: 14:35:07.123456;
• LocalDateTime.now() — the date and time: 2026-10-17T14:35:07.123456;
• ZonedDateTime.now() — the date and time with the JVM default zone: 2026-10-17T14:35:07.123456-04:00[America/New_York];
• Instant.now() — the current moment, printed in UTC: 2026-10-17T18:35:07.123456Z.

The precision of the fraction depends on the Java version: milliseconds in Java 8, usually microseconds since Java 9. To print the date in a fixed format such as MM/dd/yyyy, use LocalDate.now().format(DateTimeFormatter.ofPattern("MM/dd/yyyy")).

The no-argument now() uses the JVM default time zone (ZoneId.systemDefault()), which comes from the OS settings; it is simply now(Clock.systemDefaultZone()). If the server runs in UTC and you are in New York, the time differs by 4 hours, and late in the evening even the date is different.

• now(ZoneId) returns the time in an explicitly given zone: LocalDateTime.now(ZoneId.of("America/New_York")).
• now(Clock) takes the time from a Clock object. Production code passes the system clock, and a unit test passes Clock.fixed(Instant.parse("2026-10-18T01:00:00Z"), ZoneId.of("America/New_York")): then LocalDate.now(clock) always returns 2026-10-17, and the test does not depend on the day it runs.

Other options are starting the JVM with -Duser.timezone=America/New_York or storing moments as Instant and converting to a zone only for display.

• LocalDateTime — a wall-clock date and time without a zone: 2026-10-17T14:35:07. Use it for a meeting in one city or the time on a receipt.
• ZonedDateTime — a date, time and time zone: 2026-10-17T14:35:07-04:00[America/New_York]. Use it for flights, calls across countries and daylight saving time.
• Instant — seconds and nanoseconds since 1970-01-01T00:00:00Z, an exact moment. Use it for timestamps in logs, databases and messages between services.

No, Instant does not store a time zone: it is just a point on the UTC time-line, so toString() always prints it in UTC (the Z suffix). A time zone exists only in ZonedDateTime (a region such as America/New_York) and OffsetDateTime (a fixed offset such as -04:00). To see the local time of an Instant, you must name a zone: instant.atZone(ZoneId.of("America/New_York")).

• Immutability. Date and Calendar are changed by setTime(), set(), add(); in java.time methods such as plusDays() return a new object.
• Thread safety. The old classes and SimpleDateFormat are not thread-safe; the java.time classes, including DateTimeFormatter, are.
• Numbering. In the old API months start at 0, and new Date(y, m, d) counts the year from 1900: new Date(2024, 1, 15) is February 15, 3924. In java.time months start at 1 and there is the Month enum: LocalDate.of(2024, 2, 15).
• Validation. Calendar silently rolls Feb 29, 2023 over to Mar 1, 2023, while LocalDate.of(2023, 2, 29) throws DateTimeException.
• Separation of concepts. Date is a date, a time and a moment at once; java.time has one class per task.

Through Instant, and always with a time zone: a Date is a moment in time (milliseconds since the epoch), while LocalDate and LocalDateTime have no zone.

• Date → Instant: date.toInstant(); back: Date.from(instant);
• Date → LocalDate: date.toInstant().atZone(zone).toLocalDate() or LocalDate.ofInstant(date.toInstant(), zone) in Java 9+;
• Date → LocalDateTime: LocalDateTime.ofInstant(date.toInstant(), zone); back: Date.from(ldt.atZone(zone).toInstant());
• GregorianCalendar ↔ ZonedDateTime: calendar.toZonedDateTime() / GregorianCalendar.from(zdt);
• java.sql.Date ↔ LocalDate: sqlDate.toLocalDate() / java.sql.Date.valueOf(localDate).

Example: new Date(0L) is 1970-01-01 in UTC but 1969-12-31T19:00 in America/New_York. Convert only at the boundary with legacy code: JDBC 4.2 and Hibernate 5+ work with LocalDate, LocalDateTime and Instant directly.