I cannot say if it is a law or the pricing plan but 30 days seems to be the most recurrent job post length.

That means that the crash probably started in October and we started seeing it on the data at the beginning of November.

Looking at it again, I don't think the law idea I put forth makes sense. The pricing plan sounds more likely, especially given the ringing behaviour at multiples of 30/31 days.

I guess a large majority of those postings go unfilled.

Do you do any work to try to find if a position has been re-posted and recombine it with its previous posting for the purpose of this calculation?

I don't know if one can draw the October conclusion. Would HR leave the posting up to expire on their own if they know they won't hire anyone?

I say that since they seem to remove them (presumably because they got enough applicants or actually hired someone) before the 30/31 day mark.

Not sure which sources she crawled ... Many of the German job boards require companies to purchase job ads which expire after 30 days.

I would assume that it's not due to HR choosing to take them offline, but because that's the interval you pay for them on the platform that was scraped.