Replace formula with its value automatically

Plz see the attached workbook "Purchase and Sales Details"

which contain 2 sheets i.e. "Purchase & Sales" and "Stock".

whenever i purchase a new item i make the entry for it and enter

the corresponding date, likewise when an item is sold, its entry

is also made, hence i know when an item was purchased and when it

was sold.

Now the problem is i want to know that on a particular date how

many items were in stock which is the other sheet. which contains

Dates and the next column which i named "Stock" should contain the formula which calculates the items left.

i have tried a few formulas but whenever the new entry is made, all the formulas are recalculated, which i think excel defaults. can anyone of you gurus solve this problem for me.

Thanks in anticipation

Please some one help