I believe Insert > Date & Time (and its two companions) are intended for use in headers and footers. These are available after choosing File > Print. When used there, the inserted date can be clicked for a little popup formatter and button to manually reset the time to now.
The feature you want, an auto-generated NOW cell that does not change, is apparently not available.
Option A: Put the NOW function into a cell somewhere. It shows the current time. Copy the cell, then Edit > Paste Formula Results into the cell you want to have the unchanging time.
Option B: Use a general macro app (like the amazingly capable Keyboard Maestro) to generate your date/time text and paste it into a cell.
In both cases, choose the date and time formats you want on the final cell.
(Numbers 3.5.3 used for this answer)
This may or may not be what you are after.
This Assumes you know which cells you will be referencing. And remember this is only an example..
IF(A2<TODAY(),D2,INDEX(D,COUNTA(D) + 1))
Using the IF function.
IF(if-expression,if-true,if-false)
First Compare date value in A2 with Todays date TODAY() as the if-expression
A2<TODAY()
We use the less than symbol < to return a true or false for the expression.
If the expression returns True, then we fill the cell with The cell data in D2
If the expression returns False, then we fill the cell with the last with data in column D
The INDEX function returns the value in the cell located at the
intersection of the specified row and column within a collection of
cells or from an array returned by an array function.
.
The COUNTA function returns the number of its arguments that are not
empty.
It is bets you look up in the Functions panel for a fuller explanation on all the functions.
Once you have set up the first row use the drag handle to drag the formula cell down to the other rows you want it on.
They will auto adjust to the correct cell references.
Notes:
- At time of this posting the date was 6 September 2014
- You will need to find a way of dealing with Blank cells in the data column. If a blank cell is found it will throw the results. You should find a resolve with a google search. It will be the COUNTA function that you need to look at.
Best Answer
Go to Insert > Date & Time.
Double-click the date and format to taste. Make sure "Automatically update on open" isn't ticked.