Date Formats in Asp.net

DateTime  s = System.DateTime.Now;
DateTime  yesterday = DateTime.Today.AddDays(-1);
 Response.Write(yesterday.ToString("yyyy-MM-dd"));
  DateTime opt = Convert.ToDateTime(ontime);
        DateTime offft = Convert.ToDateTime(offtime).AddHours(24);
        string cdt = DateTime.Now.ToString();
        DateTime ctime = Convert.ToDateTime(cdt)
  string date = System.DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss")