* added some convenient functions, operator etc...
* reused parts of the patch by Stephen Deken, thanks
git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@27954 a95241bf-73f2-0310-859d-f6bbb57e9c96
* more get/ set functions, operators
* date until 1582 are handled in julian calendar
* date above 1582 are handled in gregorian calendar
git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@27264 a95241bf-73f2-0310-859d-f6bbb57e9c96