Found with https://www.google.com/search?q=WhatsApp+Date+Format I see that it's a quirky thing with no clear answer. Try the clear cache, update app ideas. And if still bugged, report this bug to the app author.
I prefer writing a date in the dd-mm-yyyy format. Why has WhatsApp messed with it to mm-dd-yyyy format? How do I reset it? I use Android phone