adding a time stamp in a cell

I have column a2 thru a20 where I add data. I want column b2 to b20 to automatically enter the date and time I entered the data in column a2,a3,.... I am using the the following formula in column b: =IF(A2>0,NOW(),IF(A2=0,"")). It work okay when I enter data in a2 The date and time is automatically entered in b2, but when I add new data at a much later time in a3 the time changes in b2 with the date and time that is auto entered in b3 when the new data was entered. How do I overcome this?
Thanks in advance.
Alex

i found this some where with a search(http://www.mcgimpsey.com/excel/timestamp.html)
You can use a circular reference to enter the time when a change is made in another cell, then maintain that time. Choose Tools/Options/Calculation and check the Iteration checkbox. Then, if your target cell is A1 and you want the date/time to appear in B1, enter this in B1:

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Format B1 as you wish to display date, time, or both. If A1 is initially blank, B1 will return a null string (""). When a value is entered into A1, B1 will evaluate as "", therefore NOW() will be returned. After that (as long as A1 remains populated), B1 will evaluate to a date/time and therefore will return the value in B1 - i.e., the date/time.

Your answer is exactly what I was looking for.
Thank you very much.

Although this may sound odd, how can I make the time stamp be yesterday's date when I add data in A1 today. I tried adding a "-1" after the now() but it does not work. Thinking ahead how can I make todays date be either past or future date.
Thanks again.