If your textbox takes date in dd/MM/yyyy format, the following code will convert it into valid datetime value:
string InputDate = YourDateTextBoxId.Text.Trim();
System.Globalization.DateTimeFormatInfo dateInfo = new System.Globalization.DateTimeFormatInfo();
dateInfo.ShortDatePattern = "dd/MM/yyyy";
DateTime OutputDate = Convert.ToDateTime(InputDate, dateInfo);