Take a look here:

http://www.mcgimpsey.com/excel/timestamp.html

In article <D9291E32-66A0-4258-96D6-22EDBE3E6B28@microsoft.com>,
John M <John M@discussions.microsoft.com> wrote:

> I am looking to entering a value into column A (Data Point), then on column B
> (Time Stamp), the time that value A was entered. Then another value will be
> entered into A2 and would like the time that Data Point 2 was entered into B2
> and so on. I have tried the IF statement =IF(A1,NOW(),"") in B1,B2, B3...
> but every time the work sheet reculculates, the Time Stamp column becomes the
> same. How can I stop this from happening or am I going about this the wrong
> way.