Regarding Daylight saving time

[vind123]

We are using HP-UX 10.2. In US, beginning in March of 2007 the starting and ending dates for Daylight Saving Time will change. Clocks will be set ahead one hour on the second Sunday of March instead of the current first Sunday of April. Clocks will be set back one hour on the first Sunday in November, rather than the last Sunday of October.
I came to know HP is not going to support for patches of HP-UX 10.2 version to the daylight saving time. Also we are not going to upgrade the OS. What can i do for this one.
If i modify /usr/lib/tztab is it sufficient? Should i change the UNIX system time for reflecting the daylight saving change. Also is HP-Unix time is GMT time? In my C programs i am able to see its using localtime function and tm_isdst flag of tm structure . I am not aware what i need to modify to reflect the daylight saving.

[Patrick Wallek]

Yes, modifying the tztab file will be sufficient.

Yes, HP-UX keeps time in GMT/UTC.

[James R. Ferguson]

Hi:

HP-UX release 10.20 is no longer supported. In the case you describe, however, that's not a problem. Simply fetch the latest 'tztab' patch (for instance, PHCO_34668 for 11.11) and replace the 10.20 version of '/usr/lib/tztab' from the patch's depot.

Having done this, plan a reboot. This will insure that all processes inherit the new ruleset.

Remember, all timekeeping is done in UTC (GMT). What a process or a user sees as the correct time is only an *offset* from UTC as dictated by the TZ variable.

The 'tm_isdst' element of the tm structure in C denotes whether or not the current time is under daylight savings or not.

The only modification that you need to make to anything is to update the '/usr/bin/tztab' rules.

Regards!

...JRF...

[A. Clay Stephenson]

and to make sure that you have correctly changed /usr/lib/tztab, you can run the attached Perl script, dst.pl. Because it uses the same underlying libc date routines that C programs use as well as other standard utilities such as date, if the Perl script works then the other utilities will also work.

Use it like this:
dst.pl -y 2007

That will display the exact second before and after the time transitions.

You can test other TZ settings by:

TZ=CST6CDT dst.pl -y 2007

Invoke as dst.pl -u for full usage.

[vind123]

Thanks a lot.
1.You had Replied "The only modification that you need to make to anything is to update the '/usr/bin/tztab' rules"

I am unable to find the tztab file in /user/bin directory. Only in /usr/lib i am able to locate it. Hope the file i am going to modify is correct one.

ll /usr/lib/tztab
-r--r--r-- 1 bin bin 9376 Mar 9 1998 /usr/lib/tztab

Through which user or command i can modify this file since it doesn't have write permission ?What i need to modify in it? After modification do i need to restart the cron and application or reboot the machine?
Also I need not change unix system time(GMT) for this daylight saving change. Hope i am correct?

2. You had replied "HP-UX release 10.20 is no longer supported. In the case you describe, however, that's not a problem. Simply fetch the latest 'tztab' patch (for instance, PHCO_34668 for 11.11) and replace the 10.20 version of '/usr/lib/tztab' from the patch's depot"

I am going for the first solution of changing tztab file since I had not done any patch installation earlier in UNIX system. I am interested in knowing about it. In which URL i can download the PHCO_34668 patch and how do i install it and replace the 10.20 version f '/usr/lib/tztab' from the patch's depot. Also how to reboot the machine.

[A. Clay Stephenson]

You don't need any special utilities to alter /usr/lib/tztab; it's simply a text file so all you need is vi. As long as you are root, permissions are ignored so that you can modify this file even without the write permission bit being set.

Attached are the 4 contiguous US timezone entries with the necessary changes. Simply cut and paste them into your existing /usr/lib/tztab using vi.

# Pacific Standard Time, Pacific Daylight Time
PST8PDT
0 3 24-30 4 1970-1973 0 PDT7
0 3 6 1 1974 0-6 PDT7
0 3 22-28 2 1975 0 PDT7
0 3 24-30 4 1976-1986 0 PDT7
0 3 1-7 4 1987-2006 0 PDT7
0 3 8-14 3 2007-2038 0 PDT7
0 1 25-31 10 1970-1973 0 PST8
0 1 24-30 11 1974 0 PST8
0 1 25-31 10 1975-2006 0 PST8
0 1 1-7 11 2007-2038 0 PST8

# Eastern Standard Time, Eastern Daylight Time
EST5EDT
0 3 24-30 4 1970-1973 0 EDT4
0 3 6 1 1974 0-6 EDT4
0 3 22-28 2 1975 0 EDT4
0 3 24-30 4 1976-1986 0 EDT4
0 3 1-7 4 1987-2006 0 EDT4
0 3 8-14 3 2007-2038 0 EDT4
0 1 25-31 10 1970-1973 0 EST5
0 1 24-30 11 1974 0 EST5
0 1 25-31 10 1975-2007 0 EST5
0 1 1-7 11 2007-2038 0 EST5

# Central Standard Time, Central Daylight Time
CST6CDT
0 3 24-30 4 1970-1973 0 CDT5
0 3 6 1 1974 0-6 CDT5
0 3 22-28 2 1975 0 CDT5
0 3 24-30 4 1976-1986 0 CDT5
0 3 1-7 4 1987-2006 0 CDT5
0 3 8-14 3 2007-2038 0 CDT5
0 1 25-31 10 1970-1973 0 CST6
0 1 24-30 11 1974 0 CST6
0 1 25-31 10 1975-2006 0 CST6
0 1 1-7 11 2007-2038 0 CST6

# Mountain Standard Time, Mountain Daylight Time
MST7MDT
0 3 24-30 4 1970-1973 0 MDT6
0 3 6 1 1974 0-6 MDT6
0 3 22-28 2 1975 0 MDT6
0 3 24-30 4 1976-1986 0 MDT6
0 3 1-7 4 1987-2006 0 MDT6
0 3 8-14 3 2007-2038 0 MDT6
0 1 25-31 10 1970-1973 0 MST7
0 1 24-30 11 1974 0 MST7
0 1 25-31 10 1975-2006 0 MST7
0 1 1-7 11 2007-2038 0 MST7

[vind123]

1. Should i stop the application and cron before modifying the /usr/lib/tztab file. After modification should i restart the application and cron. Does any rebooting of unix machine is required.

2. Pls correct if my understanding is wrong.
For daylight saving change, I need not change the UNIX system time(GMT) and only /usr/lib/tztab needs to be modified. All the C functions localtime .. and tm_isdst flag of tm structure will properly work after the modification of tztab file.

3. This program dst.pl is used only for testing the modification of /usr/lib/tztab. Pls correct me if i am wrong. Thanks a lot for the code.


4. Also is the HP-PATCH does the samething , modification of the /usr/lib/tztab?

[James R. Ferguson · Accepted solution]

Hi (again):

1. No, you do not need to stop anything before modifying '/usr/lib/tztab'. You can restart 'cron' after the modification or arrange for a server reboot some time before the changes in the daylight saving rules are to take effect. The idea is simply that old processes need to be restarted to see the new rules.

2. That is correct. You only need to change the aforementioned file.

3. Clay's script is only for testing. It modifies nothing.

4. The patch I suggested is simply one way to obtain a newly updated, coherent set of timezone rules. It is certainly valid to update your existing file as suggested, too.

p.s. Yes, I inadvertantly wrote "/usr/bin/tztab' at the end of my first post but meant '/usr/lib/tztab' as I had correctly written at the beginning.

Regards!

...JRF...

[vind123]

1. You had replied "No, you do not need to stop anything before modifying '/usr/lib/tztab'. You can restart 'cron' after the modification or arrange for a server reboot some time before the changes in the daylight saving rules are to take effect. The idea is simply that old processes need to be restarted to see the new rules"

why we need to reboot the machine before the changes in the daylight saving rules are to take effect. It should be done after the changes. Why it needs to be done before the changes? Also why application restart is not required?


2. The United States uses eight time zones. From east to west they are Atlantic Standard Time (AST), Eastern Standard Time (EST), Central Standard Time (CST), Mountain Standard Time (MST), Pacific Standard Time (PST), Alaskan Standard Time (AST), Hawaii-Aleutian Standard Time (HST), and Samoa standard time (UTC-11). View the standard time zone boundaries.


Clay had provided the 4 time zones to update in the file i.e Pacific Standard Time, Eastern Standard Time,Central Standard Time and Mountain Standard Time

I am able to find the details about other time zones.
1. ALASKAN - Not found in the /usr/lib/tztab file
2. ATLANTIC - Found in the /user/lib/tztab
3. HAWAII - Not found in the /usr/lib/tztab file
4. Samoa standard time - Not found in the /usr/lib/tztab file

I am unable to find ALASKAN ,HAWAII and Samoa in the tztab file. other than the list provided by clay i need to do the modification only in ATLANTIC and leave ALASKAN,HAWAII ,Samoa. Hope i am correct?


2. Daylight Saving Time begins at 2:00 a.m. local time on the first Sunday in April. On the last Sunday in October areas on Daylight Saving Time return to Standard Time at 2:00 a.m.

i am able to see the below entries in tztab file. why there is not an entry for 2:00 am
0 3 1-7 4 1987-2006 0 PDT7
0 1 25-31 10 1975-2006 0 PST8

[vind123]

awaiting reply

[A. Clay Stephenson]

You don't see 2 AM because it is skipped; man tztab for a full explanation. As far as I know, the new DST rules apply only to the 4 contiguous US timezones. You've already got your answers. The new tztab replacements will not be recognized by any currently running processes (e.g. cron) but any processes started after the tztab changes will see those changes. After making the tztab changes, the safest, surest approach is to reboot but that is not necessary if you know your system and it's applications well enough.

I suggest that you stop asking questions and start doing.

[vind123]

Clay, Thanks a lot for the info. Apologies for more questions. Will start doing.

[vind123]

I had assigned the points. Thanks a lot for the info.

[Bill Hassell]

It's important to understand that timezones in HP-UX have NO limitations. You can define anything you like including your own private timezone. Just assign the TZ variable with a new timezone. The /uzr/lib/tztab file is used as a convenience for the major locations. Toemzones are political and always subject to change so HP-UX handles this very simply. Read the man page "environ" and you'll see how the TZ variable can be set to anything you like:

TZ=BILLH5 date
TZ=PIGEON9 date

The TZ value is globally set for each login using the file /etc/TIMEZONE. You can edit tztab at any time and the result is immediate. Time never changes in HP-UX, it is always GMT (or UTC or Zulu). The TZ variable tells the kernel time/date libraries how to translate the time for programs and users. You can have dozens of different timezones on your system at the same time.

[Link Communications Cor]

Could anyone please explain how the file /usr/lib/tztab work in term of system? Also when changing, say from EST-10 to EST-10EDT, does the server need to reboot to take an effect for the system? If yes,Why/why not?

[Dennis Handly]

>Could anyone please explain how the file /usr/lib/tztab work in term of system?

When you use any of the ctime(3C) functions that provide local vs GMT (UTC) time, libc reads tztab to determine the timezone and when DST starts. See tztab(4) and TZ under environ(5).

>Also when changing, say from EST-10 to EST-10EDT, does the server need to reboot to take an effect for the system?

The computer works in GMT time. So it doesn't matter what timezone you are in. Unfortunately there are all sorts of demons and logs that like to print out the local time. If the latter is important to you, you'll have to reboot if you can't restart them.

As to long running user applications, once libc reads tztab at the start, it doesn't read it again unless TZ has changed. (By putenv.) So these may have to be restarted too.

Since you can't assign points, it would have been better to open a new thread and point to this one.