Yearnum


 * ISO-8601 year number
 * date to year number
 * test set
 * 2005-01-01 → 2004
 * 2005-01-02 → 2004
 * 2005-12-31 → 2005
 * 2007-01-01 → 2007
 * 2007-12-30 → 2007
 * 2007-12-31 → 2008
 * 2008-01-01 → 2008
 * 2008-12-29 → 2009
 * 2008-12-31 → 2009
 * 2009-01-01 → 2009
 * 2009-12-31 → 2009
 * 2010-01-03 → 2009