Most example namespace URIs seem to contain some combination of year/month/day in their path:
rdf: http://www.w3.org/1999/02/22-rdf-syntax-ns#
rdfs: http://www.w3.org/2000/01/rdf-schema#>
It's not obvious (to me) why it makes sense to include part of the created date in the URI when the concepts that are included in the vocabularies are not exactly temporal.
EDIT
There may be additional confusion because of old w3c (and potentially other org) notes lying around that are still high up on SEO for semantic web. For example, this note from a w3c users group recommends the use of dates in URIs.
Manageability. Issue your URIs in a way that you can manage. One good practice is to include the current year in the URI path, so that you can change the URI-schema each year without breaking older URIs.
@cygri is still correct (the link is pre-2010), just pointing this out for people that come across conflicting information.
It's not obvious (to me) why it makes sense to include part of the created date in the URI when the concepts that are included in the vocabularies are not exactly temporal.
It's a bad idea and shouldn't be done.
These namespaces are from the very early days of RDF, when good practices around URI management for the Semantic Web were not yet understood. Today, W3C uses much shorter and undated namespaces like http://www.w3.org/ns/csvw# for new vocabularies, but changing the old namespaces is not really possible given the huge amount of data and tools that are already published with these namespaces baked in.
So why did W3C think it was a good idea to include the date back then?
Because W3C includes date information in pretty much all their URIs. It's the date when the URI was allocated. It's their way of ensuring that URIs are unique and don't accidentally clash. So, all URIs that were allocated in the year 2000 have a path that starts with /2000/, all from 2001 start with /2001/ and so on. For “high-value” documents, like W3C standards, they also allocate a short alias such as http://www.w3.org/TR/html.
I suppose back then they thought that short aliases are not necessary for vocabularies, because only machines would see those URIs, and namespace prefixes would be used to hide them from view.
Today, the general wisdom is to “leave out as much as possible” when allocating URIs. So, schema.org with class URIs like http://schema.org/Person is pretty much perfect.
Most namespace URIs seem to contain some combination of year/month/day in their path
That's not really true. If a namespace URI has a date in it, it's probably a W3C URI from before 2010. Most namespace URIs don't have dates in them.