The ThreeTen project is providing a new date and time API for JDK 1.8 as part of JSR-310.
Main project for JDK 1.8
The main strand of active development for JDK 1.8 is in OpenJDK.
Backport for JDK 1.7
A backport has been provided for JDK 1.7 hosted here at GitHub. The aim of the backport is to allow developers on JDK 1.7 to access an API that is very similar to the one in JDK 1.8. The backport is NOT an official implementation of JSR-310, as that would involve many complex legal/procedural hoops.
The backport is used in projects such as OpenGamma.
This site holds reference documentation for ThreeTen and JSR-310. This supplements the Javadoc, providing a broader user guide. The documentation is applicable to both the backport and JDK 1.8 - only the package name changes.
Not every piece of functionality in the domain of date/time ended up in OpenJDK and JDK 8. The “extras” have been combined into a new project - ThreeTen-Extra - which can be used as an additional date/time jar file on JDK 8.
The old home page is still up at Sourceforge for the moment.