Try the "IsValid(DateField) function. It'll tell you if the date is valid. If you need some particular evaluation of the date itself such as the close date being greater than the open date you can use the usual if statements and comparisons....
You don't need the goto step.
You can just do
If[Fieldname <> ""]
I couldn't get either one to work. I kept getting a flash screen that it couldn't find the table.
But, I did find a work-around. I set it for if the field contained a date prior to 1/1/2099, then clear the other field. It works fine, but is not quite as elegant.
I need to find a good basic primer on scripting, something like FileMaker Scripting for Dummies.
Make sure you go to the appropriate layout first
Your script should be
Go to layout ___
Enter FInd Mode
YOUR FIND OPERATION