In essence I want a WHEN formula or eliminate the "else" part of the IF formula.
I have the number 15 in cell C1 which is conditionally generated else where in the workbook . I have the number 1 in cell B1 which generated the 15 in C1.
If I change the number in cell B1 to 2 the number in C1 changes to 25
I want to copy the number 15 in cell C1 to cell D1 when the number in B1 in 1 and have the number in D1 remain 15 when the number in B1 changes to 2.
In essence I want to change the content of D1 only when the content of B1 meets a specific condition.
=WHEN(B1=1,COPY(C1))

If formulas need to have an else statement. If you write a VBA statement you can skip the Else statement but the complication goes up and you will want to get ready to learn.
I think you can accomplish what you need with a formula.
The formula that I would use if it were me would be
=if(B1=1,C1,"")
This will keep the formula cell blank unless the condition is met.

Excelled, I had to read this a couple times but I think I disagree, not with your solution but with your reading of agent007's goal.

Agent007, excelled is right that the IF function requires a value both for the True and the False condition. The first couple times I read your description, I wanted to ask "What's in D1 in the first place?" Say D1 is normally =A5*PI(). Then, if you want it to be A5*PI() normally but to have C1's value whenever B1=1, then you should put this in D1:But after another reread, I decided you're saying this: You want the value in D1 to be whatever it is — whatever it was before — but when B1 changes to 1 you want D1 to change to the value of C1 and stay that way even if B1 changes again.

If that's what you want, then I don't think you can get there with formulae. You need a VBA macro. Are you up for doing that?

Hmmmmn, if that's what he wants I do know of a way to do it with formulas only but you end up using autoshapes to create the illusion that the value is in the cell until you change it. It works really well for the right application but it may be more desirable to use VBA instead.

Let me restate the problem.
I have Jan, Feb, and Mar date that I process for each month separately through a set of equations. I want to save the processed data for each month separately for processing into a quarterly report. How do I save each months data for later use? I know how to copy and paste special. I know I could use copies of the processing equations for each month separately but that consumes processing resources. Is there another way to save each months processed data for later use?

Well, if you know how to copy and paste, why not do that? It's only once a month, right? You could write a program to do it, but for one copy/paste per month it doesn't sound as though it'd be worth the trouble.

Probably that means I've misunderstood the requirement.