This indeed was way too easy.
Because I made it easy enough to be cracked, hence the puzzle.
So, even [...] if it wouldn't be purely based on security through obscurity (which it does), it still would be a worthless scheme.
It's not. It's not
cryptographically secure, but it's still quite secure, depending on how you use it:
With 2 dates in 1900-2021 range there are about 1 billion possibilities. With 3 dates it's 14 trillion, with 4 dates it's 158 quadrillion. Good luck cracking that.