Speaking of
real-world knowledge, this might be a good place to note the source of the sentence. It didn't just spring into existence as a
Computer Science tagline.
光陰矢の如 し
"Time flies like an arrow" is an old Japanese proverb. In Japanese it's "Kouin yanogotoshi" - which, literally translated, actually means "time is like an arrow".
I suppose the closest English equivalent would be "time and tide wait for no man". However, it seems that the Japanese version is (now, at least) much more commonly used. You can see it in a lot of people's letters and and online diaries. Of course, a simple "time flies" would do, but it isn't as descriptive. The arrow simile, furthermore, implies not only time's speed, but its irreversibility as well. The arrow cannot be pulled back to the bow, and time, once lost, can never be regained. Terminator movies notwithstanding.
-- Many thanks to sekicho for help with the Japanese.