Dates are very much a [local thing](https://en.wikipedia.org/wiki/Date_format_by_country) so really thats why the custom one is there. The fixed formats are the personal preferences of the contributors who added them, YMMV. A PR that added ISO formats will probably not be controversial (order apart :-) and would likely be accepted.
But unless the document being edited is being created from scratch any dates should be the format used elsewhere in the document, not the editors preferred.
That is the trouble with using locale (in general, this in particular) it makes software behave as if it only ever handles documents that are local, I mean how often do things get shipped to other countries ... oh the internet ... oh github :grin: [end rant]