String was not recognized as a valid date format

I have a DateTime declared in my SQL Database, DAL 
SQL: BDay DateTime
DAL:
public int InsertProfile(DateTime bday)
{
   .....
   cmd.Parameters.Add("@BDay", SqlDbType.DateTime).Value = bday;
   return result;
  ......
}
Code behind:
 result = pBAL(Convert.ToDateTime(tbdate.text)....
My primary concern here I already converted my string (textbox) to DateTime however I am getting an error "String was not recognized as a valid date format"
My thread previously has not been resolved yet, so please I need someone who give me the correct syntax on how to avoid this error in the future.

Thanks.
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