Retrieve format of date from date value
DateTime result;

if (DateTime.TryParse(value, out result))
{
    // use result here
}

select Convert(varchar(10),StartDate,103) as StartDateformat from table where filtername=@filtername

DateTime todateformat = Convert.ToDateTime(txttodate.Text.Trim());

string startdate = todateformat.ToString("dd/MM/yyyy hh:mm:ss");