How do we know that Mr Zagórski was born on 4th April?

This daily date is only the date when the post on site Paczka dla Bohatera was added. He could have been born in any other month.

It isn't written anywhere that he celebrated his birthday in April.

Mr Zagórski was born on 28 October 1918 and passed away on 10 December 2024.

The above dates were confirmed in the cemetery grave finder: https://www.zck-krakow.pl/funerals

It means that he has never been the oldest living men in Poland.