1. self代表的是调用方法的时间NSDate类型.这是在NSDate的类目中增加的方法

//获取当前的年
-(int)year {
    NSCalendar *gregorian = [[NSCalendar alloc]
                             initWithCalendarIdentifier:NSCalendarIdentifierGregorian];
    NSDateComponents *components = [gregorian components:NSCalendarUnitYear fromDate:self];
    return [components year];
}


//获取当前的月
-(int)month {
    NSCalendar *gregorian = [[NSCalendar alloc]
                             initWithCalendarIdentifier:NSCalendarIdentifierGregorian];
    NSDateComponents *components = [gregorian components:NSCalendarUnitMonth fromDate:self];
    return [components month];
}


//获取当前的日
-(int)day {
    NSCalendar *gregorian = [[NSCalendar alloc]
                             initWithCalendarIdentifier:NSCalendarIdentifierGregorian];
    NSDateComponents *components = [gregorian components:NSCalendarUnitDay fromDate:self];
    return [components day];
}


//获取当前月有多少天
- (NSUInteger)numberOfDaysInCurrentMonth
{
    // 频繁调用 [NSCalendar currentCalendar] 可能存在性能问题
   return [[NSCalendar currentCalendar] rangeOfUnit:NSCalendarUnitDay inUnit:NSCalendarUnitMonthforDate:self].length;
}


//获得一月的第一天为星期几
-(int)firstWeekDayInMonth {
    NSCalendar *gregorian = [[NSCalendar alloc]
                             initWithCalendarIdentifier:NSCalendarIdentifierGregorian];
    [gregorian setFirstWeekday:1];
    //[gregorian setLocale:[[NSLocale alloc] initWithLocaleIdentifier:@"nl_NL"]];
    
    //Set date to first of month
    NSDateComponents *comps = [gregorian components:NSCalendarUnitYear | NSCalendarUnitMonth | NSCalendarUnitDay fromDate:self];
    [comps setDay:1];
    NSDate *newDate = [gregorian dateFromComponents:comps];
    
    return (8 - [gregorian ordinalityOfUnit:NSCalendarUnitWeekday inUnit:NSCalendarUnitWeekOfMonthforDate:newDate]);
}


//减去第一周的天数，剩余天数除以7，得到倍数和余数, 获得一个月有几周
- (NSUInteger)numberOfWeeksInCurrentMonth
{
    NSUInteger weeks = 0;
    NSUInteger weekday = [self firstWeekDayInMonth];
    if (weekday > 0) {
        weeks += 1;
    }
    NSUInteger monthDays = [self numberOfDaysInCurrentMonth];
    weeks = weeks + (monthDays - weekday)/7;
    if ((monthDays - weekday) %7 > 0) {
        weeks += 1;
    }
    
    return weeks;
}

2.在调用方法的类中写方法

//上一个月的日期
- (NSDate *)lastMonthDate:(NSDate *)date
{
    NSCalendar *calendar = [NSCalendar currentCalendar];
    NSDateComponents *components = [calendar components:(NSCalendarUnitWeekday | NSCalendarUnitMonth |NSCalendarUnitYear | NSCalendarUnitDay) fromDate:date];
    if ([components month] == 1) {
        [components setMonth:12];
        [components setYear:[components year] - 1];
    } else {
    [components setMonth:[components month] - 1];
    }
    NSDate *lastMonth = [calendar dateFromComponents:components];
    return lastMonth;
}

//下一个月的日期
- (NSDate *)nextMonthDate:(NSDate *)date
{
    NSCalendar *calendar = [NSCalendar currentCalendar];
    NSDateComponents *components = [calendar components:(NSCalendarUnitWeekday|NSCalendarUnitMonth |NSCalendarUnitYear | NSCalendarUnitDay) fromDate:date];
    if ([components month] == 12) {
        [components setMonth:1];
        [components setYear:[components year] + 1];
    } else {
    [components setMonth:[components month] + 1];
        
    }
    NSDate *lastMonth = [calendar dateFromComponents:components];
    return lastMonth;
}

同上可以得到上一年,上一天等等时间