12-07-2011
I sort of agree with "achenle" mainly because unix systems which need accurate time will have additional hardware to keep time and a suite of program routines to access the alternative clock - or even accurately timestamp external events (like someone crossing the finishing line).
Systems with an ordinary "PC style" real time clock will normally be set to use NTP to keep their clocks in sync with the rest of the world. Without adjustment the clock drift can be as much as a minute-a-month.
Sometimes you do need accurate time if you are indexing real time events in a database or trying to generate a decent spread of unique filenames.
Before servers came with real time clocks the Operating System would keep track of time by counting clock ticks since boot.
Real-time clock - Wikipedia, the free encyclopedia
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LEARN ABOUT HPUX
clock_settime
clocks(2) System Calls Manual clocks(2)
NAME
clock_settime(), clock_gettime(), clock_getres() - clock operations
SYNOPSIS
DESCRIPTION
clock_settime()
The function sets the specified clock, to the value specified by Time values that are between two consecutive non-negative integer multi-
ples of the resolution of the specified clock are truncated down to the smaller multiple of the resolution.
clock_gettime()
The function returns the current value for the specified clock,
clock_getres()
The resolution of any clock can be obtained by calling Clock resolutions are implementation defined and are not settable by a process. If
the argument is not NULL, the resolution of the specified clock is stored into the location pointed to by If is NULL, the clock resolution
is not returned.
A clock may be system wide, that is, visible to all processes; or per-process, measuring time that is meaningful only within a process.
The following clocks are supported:
This clock represents the realtime clock for the system.
For this clock, the values returned by and specified by represent the amount of time (in seconds and nanoseconds) since
the Epoch. It is a system wide clock. The privilege is required to set this clock. Processes owned by the superuser
have this privilege. Processes owned by other users may have this privilege, depending on system configuration. See
privileges(5) for more information about privileged access on systems that support fine-grained privileges.
This clock represents the amount of time (in seconds and nanoseconds)
that the calling process has spent executing code in the user's context. It is a per-process clock. It cannot be set
by the user.
This clock represents the amount of time (in seconds and nanoseconds)
that the calling process has spent executing code in both the user's context and in the operating system on behalf of
the calling process. It is a per-process clock. It cannot be set by the user.
These clocks are high resolution hardware clocks
present on HP-RT realtime systems. It is included here so that applications accessing this hardware can be compiled on
HP-UX systems and then ported to an HP-RT target. HP-UX does not support or
RETURN VALUE
A return of zero indicates that the call succeeded. A return value of -1 indicates that an error occurred, and is set to indicate the
error.
ERRORS
If any of the following conditions occur, the and functions return -1 and set (see errno(2)) to the corresponding value:
The functions
and are not supported by this implementation.
The argument does not specify a known clock.
The argument to is outside the range for the given
The argument specified a nanosecond value less than zero or greater than or equal to 1000 million.
The requesting process does not have the necessary privileges to set the
specified clock.
The or argument points to an invalid address.
EXAMPLES
Advance the system wide realtime clock approximately one hour:
Get the resolution of the user profiling clock:
AUTHOR
and were derived from the proposed IEEE POSIX P1003.4 Standard, Draft 14.
SEE ALSO
timers(2), privileges(5).
STANDARDS CONFORMANCE
clocks(2)