Thanks to this diary entry, I just discovered a Wiki page Date namespace which exists there since 2014 as an improperly published proposal. Basically, it is about an introduction of syntax that supposedly gives us the ability to indicate a date range for virtually any key. Proposed syntax looks like this:

<key>:<year>-<year>=<value>
<key>:<date>--<date>=<value>
<key>:<year>-=<value>
<key>:-<year>=<value>

Unfortunately, I haven’t been aware of this until today, but it’s never too late to address it.

First of all, storing a variable value in a form of colon-delimited suffix (or prefix) is not the same as utilizing a namespace because namespace always serves a purpose of grouping. So, this proposal has nothing to do with a namespace.

The second problem is that variable colon-delimited suffix makes data processing awfully redundant. With a proper namespace suffix, it is easy to compare it with a set of known ones within a simple query, while the date “namespace” syntax requires a complex regex (including an ISO 8601 date format pattern) to find all keys containing it. There is no way to “just” select them all because this scheme does not include a qualifier of any kind. Even a bit improved syntax like <key>:daterange<year>-<year>=<value> would allow way simpler preprocessing, but it didn’t happen.

You can read more detailed explanations of these two main issues on Talk:Proposed features/Date namespace page.

I am perfectly aware of at least one web service, where developers have managed to utilize this data salad, but it only means that they had enough free time on their hands. Anyone who wants to argue is welcome to start from writing an Overpass Turbo query, showing the names of Irish counties for a specific date (say, 1920) and showing it here in comments.