Conversion failed when converting date and/or time from character string

I think you are try to convert a string which is not a proper date format thats why you are getting error in sql server, so before sending to sql please convert to datetime formate

Convert the Textbox to DateTime. Its because of convertion. C# accepts all as Strings. when you enter date, you have to convert it

 

DateTime.ParseExact(Textbox1.Text.Trim(), "dd/MM/yyyy", null).ToString("MM/dd/yyyy");