string replace_day1= your desired date.ToString("dd/MM/yyyy");
string replace_timenow=your desired date.ToString("dd/MM/yyyy");
string insert1 = "insert into tbHangNhap(DayTo,TimeNow,TenDangNhap)values(@day1,@timenow,@user)";
Sqlcommand cmd=new Sqlcommand(insert1,your connection);
cmd.Parameters.Add(new SqlParameter("@day1", replace_day1));
cmd.Parameters.Add(new SqlParameter("@timenow",replace_timenow));
cmd.executenonquery();
Another Alternative:
string replace_day1= your desired date.ToString("dd/MM/yyyy");
string replace_timenow=your desired date.ToString("dd/MM/yyyy");
string insert1 = "insert into tbHangNhap(DayTo,TimeNow,TenDangNhap)values(@day1,@timenow,@user)";
object[] pa1 = {"@day1",replace_day1,"@timenow",replace_timenow,"@user",txtuser.tex};