|As you have seen, the problem with using NOW() is that everytime the sheet recalculates, it updates every NOW() with the current "now".|
You need to lock in "now" via a VBA change macro.
For example, with this code, whenever you enter something in Column F, the cell in the same row of Column E will be set to the current date and time. The date and time will not change unless you change the value in that same cell in Column F.
Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Column = 6 Then Target.Offset(0, -1) = Now()
EDIT: This assumes that "standard" Excel VBA code runs in OOo 3. I was doing some browsing just now, and I ran across a post about needing to use something called the Listener to monitor changes to a cell in OOo 3. I don't use OOo 3, so I can't speak to that.