Why did Ainz tell Aura not to set her alarm on her watch at 19:19 and 7:21? I came across this in the Trivia section of Bukubukuchagama on Overlord Wiki:


  Giving the watch with her voice to Aura, Ainz told her not to set an alarm at 19:19 or 7:21. These numbers can be read as a sexual word in Japanese wordplay. It seems that she recorded her voice for an H-Game at that alarm. 


Since I don't know Japanese, I am curious to know how they are read.
As a comment pointed out in the wiki page you provided, the words which the numbers represent seem to be a common thing in hentai games.

For 19:19:


   In Japanese when reading the time, or a string of numbers, it is often read as if each number is separate, for example, "One Nine One Nine". Doing this with 19:19 sounds a bit like "Iku, Iku" which translates to "I'm cumming, I'm cumming!".


For 7:21:


   Thanks to ʞɹɐzǝɹ's comment we now know that it sounds a bit like "Onanī" which translates into "onanism" which is a synonym for "masturbation".


These are instances of Goroawase word play in Japanese, which takes advantage of the readings of digits to encode meaning in a string of numbers.