Precision : This is a signed integer indicating the
precision of the various clocks, in seconds to the nearest
power of two. The value must be rounded to the next larger
power of two; for instance, a 50-Hz (20 ms) or 60-Hz (16.67ms)
power-frequency clock would be assigned the value -5 (31.25 ms),
while a 1000-Hz (1 ms) crystal-controlled clock would be assigned
the value -9 (1.95 ms).