Pi seconds is a nanocentury.
This excerpted from http://www.programmingpearls.com/sec071.html
You might find it hard to memorize that there are 3.155x10e7 seconds in a year. On the other hand, it is hard to forget Tom Duff's handy rule of thumb that, to within half a percent, pi seconds is a nanocentury.
See also: RuleOfThumb, RuleOfSeventyTwo, WhimsicalUnitsOfMeasurement