Wait at least u
The actual length and variability of the time period depends
upon the implementation-defined resolution of the system clock
and upon other system and computer characteristics beyond the
scope of this standard.
Although their frequencies vary, every system has a clock.
Since many programs need to time intervals, this word is
offered. Use of milliseconds as an internal unit of time is
a practical "least common denominator" external unit. It
is assumed implementors will use "clock ticks" (whatever
size they are) as an internal unit and convert as appropriate.