好几次在项目里面做到关于日历的需求，总结了几个基本常用的方法如下：

// 获取某月某一天的公立日期
- (NSDate *)dateOfMonthWithDay:(NSInteger)day WithDate:(NSDate *)date{
    
    NSCalendar *calendar = [NSCalendar calendarWithIdentifier:NSCalendarIdentifierGregorian];
    NSDateComponents *components = [calendar components:NSCalendarUnitYear | NSCalendarUnitMonth | NSCalendarUnitDay fromDate:date];
    [components setDay:day];

    // 频繁调用 [NSCalendar currentCalendar] 可能存在性能问题
    //    NSCalendar *calendar = [NSCalendar currentCalendar];
    NSDate *dateOfDay = [self.currentCalendar dateFromComponents:components];
    return dateOfDay;
}

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

//获取当日是周几
-(NSInteger)weekdayFromDate:(NSDate *)date {
    NSCalendar *gregorian = [[NSCalendar alloc]
                             initWithCalendarIdentifier:NSCalendarIdentifierGregorian];
    NSDateComponents *components = [gregorian components:NSCalendarUnitWeekday fromDate:date];
    return [components weekday];
}

//获取当前月有多少天
- (NSUInteger)numberOfDaysInMonthFromDate:(NSDate *)date
{
    // 频繁调用 [NSCalendar currentCalendar] 可能存在性能问题
    //    NSCalendar *calendar = [NSCalendar currentCalendar];
    NSRange range = [self.currentCalendar rangeOfUnit:NSCalendarUnitDay inUnit:NSCalendarUnitMonth forDate:date];
    return range.length;
}


//获得一月的第一天为星期几
-(NSUInteger)firstWeekDayInMonthFromDate:(NSDate *)date {
    NSCalendar *gregorian = [[NSCalendar alloc]
                             initWithCalendarIdentifier:NSCalendarIdentifierGregorian];
    //1.Sun. 2.Mon. 3.Thes. 4.Wed. 5.Thur. 6.Fri. 7.Sat.
    [gregorian setFirstWeekday:1];
    //[gregorian setLocale:[[NSLocale alloc] initWithLocaleIdentifier:@"nl_NL"]];
    
    //Set date to first of month
    NSDateComponents *comps = [gregorian components:NSCalendarUnitYear | NSCalendarUnitMonth | NSCalendarUnitDay fromDate:date];
    [comps setDay:1];
    NSDate *newDate = [gregorian dateFromComponents:comps];
    
    NSUInteger firstDay = [gregorian ordinalityOfUnit:NSCalendarUnitWeekday inUnit:NSCalendarUnitWeekOfMonth forDate:newDate];
    return firstDay;
}

//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 = [self.currentCalendar 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];
    }
    [components setDay:1];
   
    NSDate *lastMonth = [self.currentCalendar dateFromComponents:components];
    return lastMonth;
}

//下一个月的日期
- (NSDate *)nextMonthDate:(NSDate *)date
{
    //    NSCalendar *calendar = [NSCalendar currentCalendar];
    NSDateComponents *components = [self.currentCalendar 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];
        
    }
    
    [components setDay:1];
    
    NSDate *lastMonth = [self.currentCalendar dateFromComponents:components];
    return lastMonth;
}