NSCalendar * calendar = [NSCalendar currentCalendar];
NSDate * currentDate = [NSDate date];
NSInteger week = [calendar ordinalityOfUnit:NSCalendarUnitWeekday inUnit:NSCalendarUnitYear forDate:currentDate];
NSLog(@"今天是今年的第%ld周",week);
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原文：http://blog.csdn.net/yangchen9931/article/details/48049453