Ask HN: How do RSS readers handle items missing pubDates?

I'm writing an RSS reader, and I noticed that items often miss publication dates. Regardless, other readers seem to be able to gather them anyway, I'm guessing through the actual site.

Is this generally done through web scraping, archival information, web crawler data, or something else entirely?

I'm asking because I would hate to resort to those types of methods as they are generally heavy on resources and unstable.