You can use DateTime.TryParseExact to help you to do this.
First,
using System.Globalization;
DateTimeFormatInfo dateFormat = new DateTimeFormatInfo();
dateFormat.DateSeparator = "/";
string clientInputDate = "31/01/2012";
DateTime date1 = DateTime.MinValue;
if (DateTime.TryParseExact(clientInputDate , "dd/MM/yyyy", dateFormat, DateTimeStyles.AllowWhiteSpaces, out date1))
{
}
else
{
MessageBox.Show("clientInputDate is not a valid recognised date format.");
Response.Write("clientInputDate is not a valid recognised date format.");
}
string yourDesireDateFormat = date1.ToString("MM/dd/yyyy");