Après-midi: Masculin ou feminin?

Question

Google translate says that the word après-midi is masculine, i.e.  this afternoon = cet après-midi.   But, when I enter "one afternoon", Google translate writes it as: "UNE après-midi", which sounds much better to me than "UN après-midi."  Is there some rule about this , and does it apply to any other words in French?  Or is Google translate making an error ?

Gruff Davies · Accepted Answer

Actually both forms are acceptable. Up until around 1920, the feminine form was more prevalent in French literature, but since then the masculine form has taken over and is preferred by L'Académie française.

Chris Wirt · Answer

Hi Ellen,
"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).

Ellen Sholk · Answer

Chris, your answer was helpful, thanks!

Ellen Sholk · Answer

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!

Noelia González Rueda · Answer

Both