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.
In addition to Alagaros' answer, I'll try to explain it from another perspective.
As explained in Wikipedia, Kanji is adapted from Chinese letters (Hanzi). Originally, Hanzi was written based on the shape of the represented object but then undergoes evolution with the passage of time. For example this.
Such a language habit is then evolving into using the existing characters to represent things. This is especially common with younger people (source: my Chinese lecturer). Think of it like the westerners using symbol characters to form smileys :D :P.
Now, the time 19:19 and 7:21 if written in kanji would be 一九一九 and 七二一 respectively.
一九 represents a kneeling person and a Neo Armstrong Cyclone Jet Armstrong Cannon (alias to keep this relatively Safe For Work). And 七二一 represents either a big Neo Armstrong Cyclone Jet Armstrong Cannon or a Neo Armstrong Cyclone Jet Armstrong Cannon and a manko.
