What is the date format for the "since" annotation in javadoc?

The @since tag should be used to define which version you added the method, class, etc. This is your hint to other developers that they should only expect the method when they run against a particular version of the package. I would consider these uber-important parts of the documentation if you're shipping your code as a library intended for someone else to use.

/**
 * @author    Firstname Lastname <address @ example.com>
 * @version   1.6        (current version number of program)
 * @since     1.5        (the version of the package this class was first added to)
 */

For more information visit:

http://en.wikipedia.org/wiki/Javadoc

javadoc: @version and @since

The tag @since is not used to specify date rather it is used to provide information about the last version information

/**    
* @since 1.7
*/

@since 1.1

since is followed by a number which indicates some sort of version ID. Any format would do but N.N format is preferred