IF Formula for a sheet

What precisely are you trying to accomplish with the formula? Return a number of days?

So you want to know two separate numbers or the combination of the two numbers (either subtraction or addition)?

So you are wanting to see the current status of a task that is active, so the difference between today's date and the Target Date where a positive number means you are "ok" but a negative number means you are behind.

And you are also wanting to see the static number for the difference between the Target Date and the Completion Date, where, again, positive is good and negative is bad.

Is that accurate?

The following is how you can accomplish this. It is with two separate formulas in two new Columns that I titled Days Remaining and Ahead/Behind.

Based on your screen shot I am assuming that you are using the =TODAY() to return the current date automatically. With that assumption the following formulas might work.

For Days Remaining:  =SUM([Target Date]3 - [Todays Date]3) - This is used to show you the current days remaining or behind on a specified task that is ACTIVE.

For Ahead/Behind:  =SUM([Target Date]3 - [Completion Date]3) - This is used to show the static day(s) that a task was complete ahead of, on time or behind the specified Target date.

If I was understanding your request correctly, this will give you the information you are looking to capture.

I attached a screen shot of the Sheet as I created it, matching your original Columns and Rows and adding the two new Columns.

Not a problem. I enjoy passing along what I have learned as I trudge through SmartSheet and it's capabilities.