https://github.com/discourse/discourse/blob/7eb5f783438bc0ea86fe22ffe6f2bb6c7d369d73/app/views/user_notifications/digest.html.erb#L115
Which is defined in
https://github.com/discourse/discourse/blob/0dee603ffca1c733e9fdcfcb18a45471e09d4872/app/mailers/user_notifications.rb#L132-L138
Which means you’ll have to change the short_no_year strings to change the date format in digests.
short_no_year