Actually I think a de facto convention I've seen often is that `mm/dd/yyyy` uses slashes, `dd.mm.yyyy` uses dots, and `yyyy-mm-dd` uses hyphens; I've rarely seen `yyyy mm dd` use anything other than hyphens. (Plus, as pointed out, ISO 8601 says to use yyyy-mm-dd with hyphens.[^1]) So it might be a good idea to follow that /.- convention here too. [^1]: Then again, ISO 8601 also says to use that ugly `T` for separating date and time, like `2023-12-31T23:59`, but nobody seems to follow that and instead prefer to use a space unless aiming for strict ISO compliance.
I don't know if ISO _should_ be the first available option, but it should definitely be an option (probably replacing yyyy/mm/dd). Personally I think what makes the most sense is to have your system/locale's default as the first option. (And if it's not possible to do it based on locale, I'd consider using the "international" and unambiguous format `yyyy-mm-dd` as the first option).