string was not recognized as a valid datetime in Asp.Net

DateTime.ParseExact which parses a date that exactly matches a format you supply.

string date1 = "12/29/2011";

        DateTime date2 = DateTime.ParseExact(date1, "MM/dd/yyyy", null);

null means the current culture.

 OR

using System.Globalization;

date2 = DateTime.ParseExact(date1, "MM/dd/yyyy HH:mm:ss", CultureInfo.InvariantCulture);