Assuming that the data in your DB is stored as a DateTime column, then it should just be a case of casting it:
emps.Add(new Employee((string)dr["First_Name"], (string)dr["Last_Name"], (int)dr["Age"], (int)dr["Earnings"],(DateTime)dr["Date_of_registration"]));
If it isn't then you need to look at what is stored in your DB, as there is a major problem!