Joda-Time provides a library of classes to replace the Java JDK Date and Time classes including formatting. It is based around the ISO8601 datetime standard, but also provides full support for other calendar systems, such as Gregorian and Buddhist.
Project moved to GitHub: https://github.com/JodaOrg/joda-time
ThreeTen provides a modern date and time library for Java and is the reference implementation for JSR-310. It includes many of the lessons of the Joda-Time project and aims to standardise date and time concepts in computing. Code now integrated into OpenJDK, this project is archived.