You didn't get it.
First, you could just write that down.
"Dear mother, decrypt the following thing by pasting it into the software called XXX on my PC: ..."
Second, that is not what i wrote.
Your secret data you have used for the shift cipher were some dates.
You could use exactly these dates (the secret information) as a key in an AES cipher. That would be already way more secure than your approach since it wouldn't leak anything about the plaintext at all.
And when decrypting, that is exactly the same effort (Taking secret info X and doing Y).
I think it's you who didn't get it... With my method you don't have to rely on any external software or use online services (and risk theft) to decrypt anything, you can do it by hand. And with my method you can simply write down 12-24 BIP-39 words, not random gibberish 100-300 characters. The point is to write them down on a piece of paper, not store them on a computer in crypto.txt that my mother has access to and can simply copy-paste it. That's just extra risk right there.