I've done this before but can't remember how i got there.

I have two columns (A and B). I have to enter a date in Column B and this
needs to automatically enter the number 1 into its neighbouring cell in
Column A. For instance if B6 had a date in it then A6 would show the number 1.
Sounds simple but Column A feeds into other spreasheets so if its not filled
in other data will be wrong. By having it filled automatically will hopefully
eliminate human error.

Thanks for your help