Design of the Java Platform Time System

The Java platform defines its time origin as the UTC epoch 1970‑01‑01T00:00:00Z, represented by the word epoch and stored internally as a long counting milliseconds from that point.

Basic usage with Date shows how to obtain the current instant and its millisecond value:

Date now = new Date();
System.out.println(now);
System.out.println(now.getTime());
System.out.println(System.currentTimeMillis());

Printing the result yields a human‑readable date and the same millisecond count. For dates before 1970 the offset becomes negative; the article demonstrates this with a Calendar set to 1969, producing a negative millisecond value.

Timezones are introduced as offsets from UTC. The article shows how to create three calendars for Shanghai, London, and Chicago using TimeZone.getTimeZone, prints their local times, and then prints the identical millisecond offsets, confirming that the offset itself is independent of the displayed zone.

Java 8 replaces raw offsets with the more expressive ZoneId and ZoneOffset. Examples illustrate creating zone IDs ( ZoneId.of("Asia/Shanghai"), etc.) and offsets ( ZoneOffset.of("+8"), ZoneOffset.of("-6"), ZoneOffset.of("+01:30"), etc.), including minute and second precision.

The article then discusses historical timezone changes, especially in China (multiple overlaps and gaps between 1901 and 1991) and the United States (regular daylight‑saving adjustments). It explains how Java stores these changes as transition rules and how to query a zone’s offset for any instant.

All core Java 8 date‑time classes are introduced with their internal fields and typical usage: LocalDate, LocalTime, LocalDateTime, OffsetDateTime, OffsetTime, ZonedDateTime, ZoneOffset, Instant, Duration, and Period. Each class diagram is described, and code snippets such as LocalDateTime.now(), LocalDateTime.now(ZoneId.of("America/Chicago")), and conversions between zones are provided.

Common operations are summarized: obtaining the current time with now(), constructing instances with of(), parsing with parse(), formatting with format(), accessing fields via get(), comparing with isBefore() / isAfter(), adding/subtracting with plus() / minus(), adjusting fields with with(), and attaching additional information with at(). The API is consistent across classes, making it far more convenient than the legacy Date API.

Finally, the author recommends using the Java 8 date‑time API for new projects, preferring local types ( LocalDateTime, LocalDate, LocalTime) and handling timezones only at the UI layer, while noting that ORM frameworks may still require the old Date type.