Steve,
You need to use a worksheet change event to do that: for example, for a change made to
any cell in column B, the date when the entry is made or changed is stored in column A
using this code:
Private Sub Worksheet_Change(ByVal Target As Range)
Dim myCell As Range
If Intersect(Target, Range("B:B")) Is Nothing Then Exit Sub
Application.EnableEvents = False
For Each myCell In Intersect(Target, Range("B:B"))
Cells(myCell.Row, 1).Value = Now
Cells(myCell.Row, 1).NumberFormat = "mm/dd/yy hh:mm:ss"
Next myCell
Application.EnableEvents = True
End Sub
Copy this code, right-click on the worksheet tab, select "View Code" and
paste the code in the window that appears.
HTH,
Bernie
MS Excel MVP