using System;

public class Program

{

    public static void Main()

    {

        string mask = GetMaskFromDB();

​

        string username = "abc";

        DateTime dt = DateTime.Now;

​

        var result = string.Format(mask, username, dt);

        Console.WriteLine(result);

    }

    private static string GetMaskFromDB()

    {

        return "{0}/{1:yy}";

    }

}