It’s not Obsidian. Try this on Windows 10:
I found that page by opening the console; the definition looks like
input[type='datetime-local']
My system’s time/date settings were set to Swiss German defaults, DD.MM.YYYY, Obsidian still showed it as MM/DD/YYYY, which is a pain to read for me.
My Windows Language was set to “English (United States)”. I changed it to “Deutsch (Schweiz)” and everything is fine.
Since you won’t be able to fix Windows, I would suggest to let people define their own format. Hey, it’s Obsidian, right? 