C++
vector<string> split(string str, char delimiter) { stringstream ss(str); string element; vector<string> split_str; while(getline(ss, element, delimiter)) { split_str.push_back(element); }

    return split_str;
}

string convert(string date)
{
    vector<string>split_date = split(date, '-');
    string formattedDate;
    string year, month, day;
    if(split_date[0].length() == 4)
    {
        year = split_date[0];
        month = split_date[1];
        day = split_date[2];
    }
    else
    {
        month = split_date[0];
        day = split_date[1];
        year = split_date[2];
    }
    if(year.length() == 2)
    {
        year = "20" + year;
    }
    if(month.length() < 2 && atoi(month.c_str()) < 10)
    {
       month = '0' + month;
    }
    if(day.length() < 2 && atoi(day.c_str()) < 10)
    {
       day = '0' + day;
    }

    formattedDate += (year + '-' + month + '-' + day);
    return formattedDate;


}

int main(int argc, const char * argv[]) {
    if(argc != 2 )
    {
        cout << "Program requires one argument (a filename) to run." << endl;
    }
    else
    {
        ifstream infile;
        infile.open(argv[1]);
        if(!infile.is_open())
        {
            cout << "Error: file not opened." << endl;
        }
        else
        {
            string line;
            vector<string> lines;
            while(getline(infile, line))
            {
                string cleanedLine;
                for(int i = 0; i < line.length(); i++)
                {
                    if(line[i] == ' ' || line[i] == '-' || line[i] == '/')
                    {
                        cleanedLine += '-';
                    }
                    else
                    {
                        cleanedLine += line[i];
                    }

                }
                lines.push_back(cleanedLine);
            }
            infile.close();
            for(int i = 0; i < lines.size(); i++)
            {
                string formattedDate = convert(lines[i]);
                cout << formattedDate << endl;


            }
        }

    }
    return 0;
}