pOS Time functions & concept - 07/27/03
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pOS provides basic functionality for date and
time. However the kernel maintains the time
not directly but it increments a kernel-variable
periodically every n-th interrupt (the n may
vary from machine to machine). But the
kernel provides functions to convert this
tick-value into milliseconds which can be
used to determine the uptime of the machine.
For more information the ticks-mechanism
see sched.txt.

In order to use real date and time, the kernel
provides the function k_settime to set the local
time. Any user process can call k_gettime anytime
to request the current system-time then.

The s_kernel struct has two member variables for
date & time functionality:

- unsigned long ticks - ticks since uptime
- s_time starttime - the real date of the
  system start, this is initially always
  set to January, 1st 2000, 00:00 but it
  is changed when k_settime is called
  (see below)

k_settime (struct s_time newtime):

1. get uptime-ticks from kernel, convert to msecs
2. deduct uptime from newtime
3. store newtime-uptime to k_kernel.starttime

struct s_time k_gettime ()

1. get uptime-ticks from kernel, convert to msecs
2. add uptime to k_kernel.starttime
3. return k_kernel.starttime + uptime