You should encode the datestring. Just replace the "/" with "%2F"
Page 1:
string s = date.ToString();
s = s.Replace("/", "%2F");
Response.Ridirect("confirm.aspx? &tariff_id= " + tariff_id + " &total_price= " + total_price+ " &date= " +s);
Page 2:
string s = Request.QueryString["date"];
s = s.Replace("%2F", "/");
DateTime dt;
DateTime.TryParse(s, out dt);
and vice versa when you read the string to build your SQL query.
--Amit