convert from DATETIME to Format("{0:dd, MMMM, yyyy} from

First you need to convert the date from a string into a DateTime type, then you can convert it to the string representation you want.

string dateString = SqlCmd.Parameters["@ZESTABLISH"].Value.ToString();
string currentFormat = "MM/dd/yyyy";    // No idea what your current format is.
DateTime dt = DateTime.ParseExact(dateString, currentFormat, null);

txt_Establish.Text = dt.ToString("dd, MMMM, yyyy");  // Or whatever format you want

Your dt must be dateTime

DateTime dt = DateTime.Parse(System.DateTime.Now.ToString());
String dates = String.Format("{0:dd, MMM, yyyy}", dt); 

Refer this :

http://www.csharp-examples.net/string-format-datetime/[^]

var dt = SqlCmd.Parameters["@ZESTABLISH"].Value.ToString();
          txt_Establish.Text = String.Format( dt,"{dd, MMMM, yyyy}");

your format should be like following

Format(Convert.ToDateTime(txtcreditsexp.Text), "dd-MMM-yyyy HH:mm")