NSDateComponents *compt = [[NSDateComponents alloc] init];
[compt setYear:2015];
[compt setMonth:12];
[compt setDay:15];
NSCalendar *calendar = [NSCalendar currentCalendar];
NSDate *date = [calendar dateFromComponents:compt];
NSUInteger count = [calendar ordinalityOfUnit:NSCalendarUnitWeekOfYear inUnit:NSCalendarUnitYear forDate:date];
count 的值爲51,就是2015年的第51周;
詳細講解請看博客:http://blog.csdn.net/felicity294250051/article/details/48496859