Apologies for the formatting, I’m writing this reply from my phone.

> And sure enough, the modification of the string is the point -- it's a
> cache! From gnus-sum.el:
>
> ; Since this is called not only to sort the top-level threads, but
> ; also in recursive sorts to order the articles within a thread, each
> ; article will be processed many times. Thus it speeds things up
> ; quite a bit to use gnus-date-get-time, which caches the time value.
> (defun gnus-thread-latest-date (thread)
> "Return the highest article date in THREAD."
> (apply #'max
> (mapcar (lambda (header) (float-time
> (gnus-date-get-time
> (mail-header-date header))))
> (flatten-tree thread))))
>
> Can we strip properties around the call, maybe?

I don’t think there’s need to do that. The change that I proposed in the patch should remove the unintended side effect (and thus fix the bug pointed by Alex) while keeping the caching behaviour intact (‘gnus-thread-latest-date’ code above will work as expected).