iOS获取当月第一天和最后一天

- (NSArray *)getMonthFirstAndLastDayWith:(NSString *)dateStr{
    
    NSDateFormatter *format=[[NSDateFormatter alloc] init];
    [format setDateFormat:@"yyyy-MM-dd"];
    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];
}