The second, minute, hour, and day handle most of life, but humanity has never stopped inventing time units for special occasions. Some are ancient and practical, some are engineering jargon, and some began as jokes that turned out to be genuinely useful. Each odd unit is a small fossil of the problem it was created to solve.
Here is a tour through the menagerie, from two weeks down to billionths of a second.
The fortnight: fourteen nights
The fortnight — from the Old English for “fourteen nights” — is simply two weeks, and it remains completely standard in British, Irish, Australian, and Indian English, while sounding quaintly archaic to most Americans. Its persistence is practical: wages, rent, and sporting fixtures often run on two-week cycles, and a single word beats saying “two weeks” every time. Counting by nights rather than days is itself a relic of older Germanic timekeeping, which also gave us “se’nnight” for one week, a word that did not survive.
The jiffy: fast, but how fast?
“Back in a jiffy” is informal English for a very short moment, but engineers could not resist pinning it down — differently in every field. In computing, a jiffy is traditionally the period of one tick of the system timer interrupt, commonly 1 to 10 milliseconds depending on the operating system’s configuration. In electronics, it can mean one cycle of the AC mains, about 16.7 or 20 milliseconds. Physicists have used it for the time light takes to travel some small reference distance, such as a centimeter. The jiffy is a cautionary tale: a unit without a fixed definition is a nickname, not a measurement.
The shake: two shakes of a lamb’s tail
The shake is the most charming of the lot. Coined during the Manhattan Project from the idiom “two shakes of a lamb’s tail”, one shake equals exactly 10 nanoseconds. The value was chosen because it is a convenient scale for the steps of a nuclear chain reaction, and the playful name doubled as informal shorthand among physicists working on very serious problems. The shake still appears in nuclear physics and related engineering fields today, a permanent reminder that even weapons laboratories name things with a wink.
Honorable mentions include the microfortnight — about 1.2 seconds, beloved of computing folklore — and the galactic year, the roughly 230 million Earth years the Sun takes to orbit the galaxy’s center.
Whenever you meet a strange unit of time, ask what job it was invented to do — the answer is usually a better story than the number itself.
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