日曆(NSCalendar)

    //日曆
    static const NSCalendarUnit CalendarUnitYMD = NSCalendarUnitYear | NSCalendarUnitMonth | NSCalendarUnitDay;
    NSCalendar *calendar = [NSCalendar currentCalendar];
    //計算當月第一天
    NSDateComponents *component = [calendar components:CalendarUnitYMD fromDate:[NSDate date]];
    [component setTimeZone:[NSTimeZone timeZoneWithName:@"UTC"]];
    component.day = 1;
    NSDate *currentDateFirstMonthDay = [calendar dateFromComponents:component];
    NSLog(@"%@", currentDateFirstMonthDay);

    //計算當月最後一天
    component.month++;
    component.day = 0;
    NSLog(@"%@", [calendar dateFromComponents:component]);

    //計算相對日期
    NSDateComponents *offsetComponent = [[NSDateComponents alloc] init];
    offsetComponent.day = 50;
    NSLog(@"%@", [calendar dateByAddingComponents:offsetComponent toDate:currentDateFirstMonthDay options:0]);

    //日期對應的星期
    NSArray *weekDaySymbols = [[[NSDateFormatter alloc] init] shortWeekdaySymbols];
    NSDateComponents *dateComponents = [calendar components:NSCalendarUnitWeekday fromDate:currentDateFirstMonthDay];
    NSLog(@"%@", weekDaySymbols[dateComponents.weekday - 1]);

    //每個月的週數，iOS8以下使用NSWeekCalendarUnit代替NSCalendarUnitWeekOfMonth
    NSRange rangeOfweeks = [calendar rangeOfUnit:NSCalendarUnitWeekOfMonth inUnit:NSCalendarUnitMonth forDate:currentDateFirstMonthDay];
    NSLog(@"%ld", (long)rangeOfweeks.length);

    //計算日期間的天數，通過修改NSCalendarUnit可以間隔的月、年等等
    NSLog(@"%ld", (long)[calendar components:NSCalendarUnitDay fromDate:currentDateFirstMonthDay toDate:[NSDate date] options:0].day);

控制檯輸出

2016-01-26 23:11:27.038 Test123[6502:707948] 2016-01-01 00:00:00 +0000
2016-01-26 23:11:27.038 Test123[6502:707948] 2016-01-31 00:00:00 +0000
2016-01-26 23:11:27.038 Test123[6502:707948] 2016-02-20 00:00:00 +0000
2016-01-26 23:11:27.039 Test123[6502:707948] 週五
2016-01-26 23:11:27.039 Test123[6502:707948] 6
2016-01-26 23:11:27.040 Test123[6502:707948] 25