Date2weekdate


 * date to weekdate
 * ISO-8601
 * YYYY-MM-DD → YYYY-Www-D
 * test set
 * 2005-01-01 → 2004-W53-6
 * 2005-01-02 → 2004-W53-7
 * 2005-12-31 → 2005-W52-6
 * 2007-01-01 → 2007-W01-1
 * 2007-12-30 → 2007-W52-7
 * 2007-12-31 → 2008-W01-1
 * 2008-01-01 → 2008-W01-2
 * 2008-12-29 → 2009-W01-1
 * 2008-12-31 → 2009-W01-3
 * 2009-01-01 → 2009-W01-4
 * 2009-12-31 → 2009-W53-4
 * 2010-01-03 → 2009-W53-7