code: function( d ) {                                                         // ISO-8601 year number. This has the same value as Y, except that if the ISO
  				var m = d.getMonth(), w = DATE_PROTO.getISOWeek.call( d );              // week number (W) belongs to the previous or next year, that year is used instead.
  				return ( d.getFullYear() + ( w == 1 && m > 0 ? 1 : ( w >= 52 && m < 11 ? -1 : 0 ) ) );
  			}
docs: Whether it's a leap year