Beware. The Time Of Day (TOD) and interval time are not the same thing.

At the end of each June and December, the International Earth 
Rotation Service (IERS) can declare a "leap second" to be added or 
deleted from UTC, resulting in 23:59:60 or an apparent jump from 
23:59:58 to 00:00:00.

UNIX has traditionally kept Greenwich Mean Time (GMT, the precessor 
to UTC) in the kernel for TOD, and converted all printed timestamps 
to local time zone in routines found in libc (e.g. ctime(3)). V7 UNIX 
even had the local time zone offset from GMT in the kernel, too, but 
that was just a convenient place to store it so that libraries and 
applications could get it.

The NTP daemon makes constant adjustments to the TOD on the systems 
where it is running (mostly by speeding up or slowing down the 
counting of clock ticks, as deeemed necessary by the calculation of 
the drift of the local system clock against reference time servers 
out on the Internet), to keep the value returned by gettimeofday(2) 
as accurate as possible.

Thus, taking a TOD timestamp, doing something, taking another TOD 
timestamp, and subtracting may not give you an accurate measurement 
of the *interval* between those two timestamps, if the TOD has been 
adjusted in between.

Dr. David Mills, inventor of NTP (among many other things) has said 
that it is very important for OS designers and application 
programmers to keep TOD and interval time separate (and be clear on 
which one you want at any given point), because they have, at best, a 
weak link.

	this moment in time brought to you by,

	Erik <fair@clock.org>