Curious, why does the date of the pdf matter? Not saying it applies here, but typically results in mathematics (unlike those in tech industry) do not become out

Curious, why does the date of the pdf matter? Not saying it applies here, but typically results in mathematics (unlike those in tech industry) do not become outdated and whatever is true today in mathematics is true tomorrow. So, for instance if you proven a mathematical theorem in 1661, then that theorem is still true today.