ThreeTenBP

ThreeTenBP, short for ThreeTen Backport, is an open-source backport of the Java 8 date and time API (JSR-310) designed for older Java platforms such as Java 6 and Java 7. It provides the core date-time types and formatting facilities of the java.time package in the org.threeten.bp namespace, including LocalDate, LocalTime, LocalDateTime, ZonedDateTime, Instant, Duration, and Period, along with ZoneId, ZoneOffset, and related classes.

The project was created to enable developers to use the modern, immutable, and well-structured date-time API before Java 8 became widely available. It mirrors the java.time API as closely as possible, allowing source compatibility while running on earlier runtimes. ThreeTenBP is organized to work with common Java build tools and is distributed via standard repositories, enabling easy inclusion in projects targeting Java 6 or 7.

Key design goals include immutability, thread-safety, and ISO-8601 compliance, with a consistent approach to time zones, calendars, and temporal arithmetic. The API surface includes not only the basic date-time types but also formatting support (DateTimeFormatter and related builder utilities) and temporal concepts such as ChronoField and ChronoUnit.

ThreeTenBP has influenced subsequent developments in Java time libraries for older environments and has a related Android-focused variant, ThreeTenABP, that adapts the backport for Android devices. Overall, ThreeTenBP serves as a practical bridge, enabling developers to adopt a modern date-time API on platforms where the official Java.time implementation is not available.