It looks more like security through obscurity.
As I wrote on my github:
Note that the encrypted words/numbers are not cryptographically secure, as they can be bruteforced to get the original words, but they do give you some protection from the common thief and some extra time to react in case of theft, etc.
Is the above true? Yes. Is it safer than writing it down in plain text? Yes.
No. It was "safer" before you published it. Now it's no more. The obscurity is gone.
Way "safer" would be to use the dates as an additional passphrase, maybe as text and together with other words. This way you wouldn't need additional software, it already works, not only with BIP39, but electrum seeds as well.