Now this is a clock for math whizzes! The clock sports math equations involving the number 9 and is made by the Triple Nine Society, a high-IQ organization:
Each number is expressed as a calculation involving three instances of the number 9.
For example, 5 o'clock is the square root of nine (3), factorial (3x2x1 = 6), minus 9/9 (6-1 = 5).
The trickiest time is 7 o'clock, whose calculation works out to 6.99999..., with an infinite number of nines. Wikipedia assures us that 0.99999... really does equal 1, so no worries that the clock is cheating there.
Link - Thanks bactac!