The annum unit is used in Wikidata statements to represent half life of isotopes, this is shown by this query. 1 annum = 1 gregorian mean year = 31 556 926 s The value is also cited by the NASA in this document as used currently for calendar calculation by them.
This unit is used in the dataset used to source these half-life statement, as shown in this document (if I’m not wrong) in the name « tropical year ». The number of seconds indicated in the item and of the « gregorian mean year » used in the document corresponds.
Amongst the different units used to represent half life statements of isotopes, it’s basically the only one that is not normalized as shown by this other query
This basically blocks the possibility to use normalization in this query that is a part of a work in progress work to build a Wikipedia Module that uses Wikidata data to maintain a frwiki table of isotope from Wikidata data. (The workflow of which would be, generate a json dataset from the query service, use listeria bot to import this in an frwiki, use Lua to load the json file from this page. This works, as a prototype, actually — see the module and a page where it’s used). It’s a comfort in the query to use the normalization possibility not to have to deal with the unit. Currently the isotopes with high life life are not colored in the sandbox page due to this.
There is a difficulty with the « tropical year » unit, it’s that in the broader sense it’s a variable quantity. But it’s not a problem if we use several items with different length in seconds and decide, as it’s done in the paper cited above, that the number of seconds is that one. Other items can eventually represent a different number of seconds for different tropical years (like the « tropical year 1900 » mentioned in the paper).
Of course it’s possible to convert without normalization one way or another, but considering the legitimate use in datasets, the importance of the unit, I think it’s worth adding it to the conversion table anyway as another variant of « year », even if the number of uses is not that high.
Request : add the item gregorian mean year to the translation table to be able to use this unit. This is NOT the « annum » unit used in the statements that motivated the query because the linked Wikipedia articles cite different possible lengths for the year definition, so a fresh item was created.