Not ``WW-1`` , but ``\W`` then ``W`` then ``-1`` .
The ``\W`` means than I really want the letter W to be shown,
the ``W`` (not escaped) means that I want the week number,
the ``-1`` means than I want the -1 to be written.
The result of ``Y-\WW-1`` is, for example, ``2014-W05-1``, which represents Monday of the 5th week of 2014.
:-)