This is Ian Darwin’s repackaging of Hirondelle Systems’s Date4J API.
I think of date4j as "the small, simple, sensible Date API for Java."
Another take: "One class to date them all". Literally, one class for almost everything.
Read all about it on http://date4j.net/
But do note that the problems that lead to the creation of date4j have been solved in the current release of Java! The Java 8 "new date/time API" (JSR-310, java.time) has what you need; see https://docs.oracle.com/javase/tutorial/datetime/
Find the current version in Maven Central for use in Maven, Gradle, Buildr, Ivy, Grape, etc. Note that the Maven package is hirondelle-date4j.
The code’s original author is not a fan of Maven, but since he has thoughtfully put it under the BSD license, this is my "Redistribution and use in source and binary forms" with an intent to make it available to all Maven/Gradle/Buildr/etc Nexus/MavenCentral users.
I have made no code changes, and plan to make none.
I have reorganized the code into standard Maven directory structure, added the POM file, removed the lib directory (uses a Maven depends), and set it up for inclusion in Maven Central and uploaded it (URL above).