an IF statement problem is there a sign or symbol I can use to correct this error

[Blobby]

we have a sheet that we use on a daily basis.
we cut and paste the cells to the left of the formula but want the formula to continue working with the cells that are pasted there.
the formula basically checks the time we have input and says whether the actual time we have input is on - time or late with conditional formating this also changes the color.
the formula we use is =IF(AA4="","",IF(Z4-AA4>=0,"On Time","Late")) is there a sign we can add or edit to make this happen?

[zbor]

Use in CF =$AA4="Late" and color you want

[Blobby]

Sorry the cf works as is.
it's the "on time" / "late" bit that doesn't work.
These cells are proteced to stop our dispatchers from deleting the formula's. (this maybe causing the problems but I need a solution?) Please