IOS 获取当月第一天和最后一天

NSDateFormatter *formatter =[[NSDateFormatter alloc] init];
[formatter setDateFormat:@"yyyy-MM"];
NSString *currentTime2 = [formatter stringFromDate:[NSDate date]];

NSArray * arr =[self getMonthFirstAndLastDayWith:currentTime2];

//方法
- (NSArray *)getMonthFirstAndLastDayWith:(NSString *)dateStr{
    
    NSDateFormatter *format=[[NSDateFormatter alloc] init];
    [format setDateFormat:@"yyyy-MM"];
    NSDate *newDate=[format dateFromString:dateStr];
    double interval = 0;
    NSDate *firstDate = nil;
    NSDate *lastDate = nil;
    NSCalendar *calendar = [NSCalendar currentCalendar];
    
    BOOL OK = [calendar rangeOfUnit:NSCalendarUnitMonth startDate:& firstDate interval:&interval forDate:newDate];
    
    if (OK) {
        lastDate = [firstDate dateByAddingTimeInterval:interval - 1];
    }else {
        return @[@"",@""];
    }
    
    NSDateFormatter *myDateFormatter = [[NSDateFormatter alloc] init];
    [myDateFormatter setDateFormat:@"yyyy-MM-dd"];
    NSString *firstString = [myDateFormatter stringFromDate: firstDate];
    NSString *lastString = [myDateFormatter stringFromDate: lastDate];
    return @[firstString, lastString];
}