I use Excel 2003 but, in this, I would have thought it should be the same.
Assuming you have formulae in a cell, when you amend the cell with the data in, then click Enter, or move from that cell, the cell with the formula in should then show the new figure.
So, not meaning to teach you to suck eggs etc, cell C1 has date, cell C2 has quantity, Cell C3 has formula. When you update C2, and press Enter, cell C3 should then display the new total.
If you want to update on a new line, then you can drag the formula from cell C3 to C4 (right bottom corner, and get a cross, then click and drag down). This will allow you to enter data in cells D1 and D2, and D3 should display the calculation.
Maybe I've misunderstood what you mean, though. If so, perhaps you could explain further, and I, or someone will try to help.