String to DateTime

// String to DateTime
String MyString;
MyString = "1999-09-01 21:34 PM";
//MyString = "1999-09-01 21:34 p.m."; //Depends on your regional settings

DateTime MyDateTime;
MyDateTime = new DateTime();
MyDateTime = DateTime.ParseExact(MyString, "yyyy-MM-dd HH:mm tt",
null);

DateTime to String

//DateTime to String
MyDateTime = new DateTime(1999, 09, 01, 21, 34, 00);
String MyString;
MyString = MyDateTime.ToString("yyyy-MM-dd HH:mm tt");

http://msdn2.microsoft.com/en-us/library/w2sa9yss.aspx