C# 根据身份证号计算是否成年

public static int IsAdult(string input)
{
    if (!string.IsNullOrEmpty(input) && !string.IsNullOrWhiteSpace(input))
    {
        if (input.Length != 15 && input.Length != 18)
        {
            return 0;
        }

        string birthday;
        if (input.Length == 18)
        {
            birthday = input.Substring(6, 4) + "-" + input.Substring(10, 2) + "-" + input.Substring(12, 2);
        }
        else
        {
            birthday = "19" + input.Substring(6, 2) + "-" + input.Substring(8, 2) + "-" + input.Substring(10, 2);
        }

        DateTime birthDate = DateTime.Parse(birthday);
        DateTime nowDateTime = DateTime.Now;
        int age = nowDateTime.Year - birthDate.Year;
        if (nowDateTime.Month < birthDate.Month || (nowDateTime.Month == birthDate.Month && nowDateTime.Day < birthDate.Day))
        {
            age--;
        }

        return 18 <= age ? 1 : 2;
    }
    else
    {
        return 0;
    }
}