周数(Week Number, WeekNum)计算

/* Set as calculateWeek to determine the week of the year based on the ISO 8601 definition.
   @param  date  Date - the date to get the week for
   @return  number - the number of the week within the year that contains this date */
iso8601Week: function(date) {
    var checkDate = new Date(date.getTime());
    // Find Thursday of this week starting on Monday
    checkDate.setDate(checkDate.getDate() + 4 - (checkDate.getDay() || 7));
    var time = checkDate.getTime();
    checkDate.setMonth(0); // Compare with Jan 1
    checkDate.setDate(1);
    return Math.floor(Math.round((time - checkDate) / 86400000) / 7) + 1;
},

/**
 * @param date {Date} 每周第 1天。
 * @return {Number}
 */
calculateWeek:function(date){
    var d = new Date(date.getTime()+(6*86400000)), t=d.getTime();
    var start = new Date(d.getTime()); start.setMonth(0); start.setDate(1);
    return Math.floor((t - start.getTime())/86400000/7)+1;
}