Sorry, obvious solution, but does a single '*' work?
If not, can you leave it blank? If so, print it out and fill in the '*' yourself.
If that is not possible either, try inputting "01-01" OR contact IRCC directly for their suggestion (which is what I would recommend the most). You can then attach their reply to your application.
In all situations, you should provide an explanation letter in order to be prudent.
EDIT: Also consider posting this in the correct sub-forum. You will get more fruitful responses there. Good luck.
EDIT 2: I was bored so I checked the document you were referring to. Just fyi, you can ignore the warning prompt and maintain the '*', when entering the D.O.B. I am unable to get the validation message that you reference. Have you updated to the latest version of Adobe Reader?