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.