Burial0268 are you using a language pack? Language packs usually contain the code needed to change the date format. If the language pack uses the incorrect format, you can report the issue to the language pack maintainer and they might be able to fix it.
There are a few places in Flarum where the date format is unfortunately still hard-coded, like the scrubber (right navigation on discussion).
If you want to make changes to Flarum's core javascript, you will need to setup a developer environment and re-compile the code after changes https://docs.flarum.org/extend/frontend Those changes should not be done in the vendor folder as any change will be lost when updating Flarum. Instead, you would have to create a fork of Flarum or make your changes through an extension.
