你为什么要做这样奇怪的事情：

Why do you do such weird things like:

string d = Request.QueryString["ava"];
DateTime a = Convert.ToDateTime(d);
txtEff.Text = d;
//insert.Parameters.AddWithValue("@LoanAvailableFrom", DateTime.Parse(txtEff.Text));

而不是 Convert.ToDateTime 使用 DateTime.ParseExact ，然后直接将该值用于SQL查询而不是TextBox内容：

insert.Parameters.AddWithValue（@ LoanAvailableFrom，a）;

当然，您还应该更好地命名变量。

Instead of Convert.ToDateTime use DateTime.ParseExact, and then use that value directly for your SQL query instead of a TextBox content:
insert.Parameters.AddWithValue("@LoanAvailableFrom", a);
Of course, you should also take care of better naming of your variables.