PDF modified date with xelatex

Suppose I have the following document foo.tex (MWE below). If I do $ latexmk -xelatex foo.tex I get a PDF with a created date which makes sense. For one I did this morning, it’s ‘Thu 26 Nov 2020 11:31:47 UTC’. It has a modified date of ‘Wed 31 Dec 1969 23:59:59 UTC’; presumably, this is something to do with ‘Unix time’: https://en.wikipedia.org/wiki/Unix_time. This doesn’t happen with $ latexmk -pdf foo.tex; modified = created, which makes sense to me.

Why does this happen? Do I need to fix it, and can I?

I am using Ubuntu 20.10 and TexLive 2020. I see the modified date when I open the PDF with Evince, but not with Okular.

As noted in the comments, this isn’t to do with latexmk. It happens with $ xelatex foo.tex and $ pdflatex foo.tex.

documentclass{article}
usepackage{lipsum}
begin{document}
lipsum
end{document}