Days until time change?

[Robert Comber]

Hi experts:

I have been asked to send notices indicating the numbers of days remaining until the change to standard time. Is there a command that will display when the time changes? Date doesn't seem to have an option for this.

Thanks,
Bob

[Bill Hassell]

The answer is: HP-UX never changes the system time. You are seeing a translated version of the date/time by virtue of the TZ variable. The man pages for env and tztab will show you how timezones are defined--you can even make up your own. And most important: 10 users can log on to your system, each with a different TZ setting and each may see a different date for the change. So the answer is in tztab, as long as $TZ contains an entry in /usr/lib/tztab.

[James R. Ferguson]

Hi Bob:

The rules for timezone changes can be found in '/usr/lib/tztab'.

To help you understand it, see the manpages for 'tztab' too.

Regards!

...JRF...

[Kent Ostby]

Bob -- There are rules in the tztab file for each timezone.

I generally just use google to figure out what day the time change is :-)

[Steven E. Protter]

I would suggest writing a little script, calling to http://www.hpux.ws/caljd.sh that changes the /etc/motd file

We've already changed our time in Israel, which is messing up all my communications plans for the US. I literally don't know what time it is and my PC has to be set to the Iraq time zone to keep proper time here.

Strange.

SEP

[Arunvijai_4]

You can use # env |grep "TZ"
TZ=EST5EDT

and try checking the differences.

-Arun

[Jannik]

take a look at the calculation:
http://webexhibits.org/daylightsaving/i.html

[A. Clay Stephenson · Accepted solution]

While it is quite true that the system time never changes - all known UNIX boxes simply count seconds since the Earth was created - 1-Jan-1970 00:00:00; the manner in which these epoch seconds is displayed does change. The attached Perl script will do the job for you. (I wrote it because I got tired of answering the same dumb question over and over.)

DAYS=$(dst.pl -d)
echo "${DAYS} until the next time change.")

Add the -e option to display the truncated integer value. Invoke as dst.pl -u for full usage. If you install Perl on your Windoes box, it will even work there.

[A. Clay Stephenson]

Ooops, I'm an idiot.

DAYS=$(dst.pl -d)
echo "${DAYS} until the next time change.")

should be:

DAYS=$(dst.pl -d)
echo "${DAYS} until the next time change."

I don't know who added that superfluous (and unnecessary) ')'.

[Jeff Carlin]

Clay, are you going to update your script for the extension to daylight savings time in 2007?

[A. Clay Stephenson]

No, I shouldn't have to. This Perl script uses the same underlying functions (localtime(), strftime(), etc.) that all UNIX utilities such as the date command use. Whenever, the libc patches are released or the revised tztab entries are available, the Perl script will just work. It really doesn't know anything itself about timechanges but trusts that the underlying functions do. HP-UX is unique in that it does have a user-configurable tztab file. Almost all other flavors of UNIX (or Windows) will require library patches.

[Jeff Carlin]

I always learn something from you Clay!

[Bill Hassell]

And just to add to Clay's comments, timezones are political definitions, so anyone can make up their own timezone and daylight saving rules--and change them every few years (which happens in many parts of the world). Because tztab can be instantly customized, you can create your own special timezone (or whatever the governements decide). Here's mine:

export TZ=BILLH5:21

Now when I type the date command, it offsets GMT -5 hours and 21 minutes with no daylight saving rule. I know, it's a dumb timezone but it illustrates how easy it is to change the way time is displayed. NOTE: setting the date/time always uses the current value of $TZ so make sure it is correct if you decide to experiment. And in general, setting the date and time on a production system is not a good idea. That's why NTP exists.

[A. Clay Stephenson]

As a further note, it would be rather useless to have a utility that figured out itself how the time changes. What you really want is a tool that tests the system itself --- which is just what the dst.pl script does. Moreover, you can test it for multiple TZ settings just like a real UNIX box might have many TZ's in play simultaneously:

dst.pl -n # use default TZ
TZ=EST5EDT dst.pl -n # use Eastern (U.S.)
TZ=PST8PDT dst.pl -n # use Pacific (U.S.)

In this mode, dst.pl will display the time 1 second before and the exact second of the time change (if the time actually changes). If using a TZ that does not change (e.g. GMT0) then the output is null (and a non-zero status is returned).