Just a quick feedback: Your code works, but KoboToolbox seems to have a strange bug for the skipping logic:
I can add the code (e.g. ${dob}='') in a XLS-skipping logic field of a form and it works fine.
When I leave the editor and reenter it, the field does not work anymore and the code is imported into Kobo's own editor field as dob = (empty field) and does not work anymore. So whenever I have to change something on the form, I have to reenter the XLS skipping codes and deleting the converted skipping code...
The same happens for another field where I use ${dob}!=''. I guess the conversion to check if it is empty somehow doesn't work properly.
Guess I'll have a look where I can report this bug...