[marginalia_nu]

To be fair, the Last-Modified header is very sketchy. I use it as one of the heuristics for determining the age of a website in my search engine. It's not great.

It's frequently found incorrect, both older and younger than the actual age of the document. It's a bit of a relic from back in ye olden days when websites were static .htm files in a folder, which is so rarely the case today.

It doesn't help it's also got overloaded uses via If-Modified-Since -style conditional requests.

[account42]

Plenty of websites also play with the user-visible dates on websites to game search engines - most dates shown in Google results seem to be complete garbage. I don't think Modified-Since is really worse, and it at least gives you a chance to maybe get a date for static pages.

But you are right that If-Modified-Since forces it to be a date for the complete document rather than the content, which might not be as useful to normal users for dynamic pages.

[marginalia_nu]

Yeah, my takeaway after having attempted to do so is that properly dating websites is a very hard problem. You can get Google-level-accuracy decent guesstimates relatively easily, but going beyond that is hard.