"après-midi" is definitely masculine, hence "un après-midi". I checked Google translator and you are correct in that it translates "one afternoon" as "une après-midi". But when you keep typing and ask it to translate "one afternoon in May", it will quickly switch back to "un après-midi en mai".
-- Chris (not a native speaker).
Wow, Gruff - this explains so much! I am 66 years old. My French grandmother was born in 1892, and would say “une après-midi “ back when I was a child learning French when we visited our French relatives .
This is a perfect example of how languages evolve. When even l’Académie Française allows a gender re-set, you know it’s a big deal!
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