GridView and Calender Example

Question

In the attached code i got problem that, when textbox1.text will be changed, monthCalender will be showed, n what date i select it is written to that textbox. But i am getting problem in entering today's date. and second problem is that after entering two dates in two textboxs, if i click on "GO" button gridview will be showed, which is working correctly. But in gridview my requirement is, when i enter "Quantity" cloumn as 5 and "Rate" column as 10, then automatically it will show "Amount" Column as multiplication of "quantity" And "Rate". Please go through my code and correct it.