Daylight saving time - how time is stored, ho…

see also
Time synchronization - xntpd

The why
Today at 2am, the time in New Zealand went ahead one hour.  Spring is not far away!  I also noticed that my daily and weekly reports were not run as normal.   This led to start thinking about why and how this occurred.
As mentioned above, the time went ahead one hour at 2am.  So, in theory, the time between 02:00:00 and 02:59:59 did not occur.  The reports in question run automatically at 2am.  So I concluded that the reason the reports didn't run was because the time didn't exist.  For my situation, that is OK.  It may not be acceptable for a production environment.  And a similar problem will occur when the time goes back.  I initially thought the reports might be run twice but discarded that possibility for the same reason that they were not run at all when the time went ahead.  That is, they weren't run at 2am this time, why would they run when the time goes back at 2am?  
I think that a possible solution would be to run the reports at some time outside the 1am to 3am timeslot.  Say 3:01 which will always occur and will occur only once.
The how
FreeBSD does not use the BIOS of your computer (well, it might, for just a little while when booting...).  FreeBSD maintains a kernal clock, which is always set to UTC (also known as Greenwich or Zulu time).  The CMOS clock may be set to local time.  A utility (adjkerntz) is provided which maintains the proper relations between these two times.
Changing the time
I have not tested the following steps.  They have been retrieved from the
mailing list archives
.  Like everything on this website, use it at your own risk.

su to root

tzsetup

touch /etc/wall_cmos_clock (to make sure it's synched)
You might also want to check out the man pages for adjkerntz.
Other things to read
timezone, tzsetup, /usr/src/share/zoneinfo, /usr/share/zoneinfo, adjkerntz, tzsetup.


本文来自ChinaUnix博客，如果查看原文请点：http://blog.chinaunix.net/u/4206/showart_523905.html

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