Setting the TZ Environment Variable on Linux

The Update Checklist for v2015.1 recommends setting the TZ environment variable on Linux platforms and points to the manpage for tzset. This is recommended to improve the performance of Cache’s time-related functions. You can find out more about this here:

https://community.intersystems.com/post/linux-tz-environment-variable-not-being-set-and-impact-caché

The manpage on my CentOS 7 test system (RHEL 6 says the same) has this to say:

“The tzset() function initializes the tzname variable from the TZ environment variable. This function is automatically called by the other time conversion functions that depend on the timezone.”

So how do you set TZ? How does it affect the time on a Linux server? Let’s see what we can learn:

The System Timezone --

For my test, I’m using Ensemble 2016.1 on a virtual CentOS system. First let’s check the system timezone. The system-config-date utility can show you.

What about “System clock uses UTC”? This refers to the server’s hardware clock. On a dedicated server, UTC is very common. When Linux is used in a dual-boot configuration with Windows, this is not the case (Windows uses local time for its system clock).

Since dual-boot configurations aren’t common for Cache’ and Ensemble installations, I won’t say more about this. The key ideas here are that the system timezone is set for the server’s timezone and that the hardware clock time is set correctly.

Time and Date Seen by Users –

Let’s take a look. Here’s a snippet from my Terminal session:

Looks OK. My shell process (running the date command) and my Cache’ process show the same time, except for a few seconds that I needed to type the WRITE command.

Setting the TZ variable --

Now let’s set TZ in the environment. The command to do this is tzselect. Here’s a typescript of the command output to set TZ

[ehemdal@localhost ~]$ tzselect

Please identify a location so that time zone rules can be set correctly.

Please select a continent or ocean.

 1) Africa

 2) Americas

 3) Antarctica

 4) Arctic Ocean

 5) Asia

 6) Atlantic Ocean

 7) Australia

 8) Europe

 9) Indian Ocean

10) Pacific Ocean

11) none - I want to specify the time zone using the Posix TZ format.

#? 2

Please select a country.

 1) Anguilla              19) Dominican Republic    37) Peru

 2) Antigua & Barbuda     20) Ecuador               38) Puerto Rico

 3) Argentina             21) El Salvador           39) St Barthelemy

 4) Aruba                 22) French Guiana         40) St Kitts & Nevis

 5) Bahamas               23) Greenland             41) St Lucia

 6) Barbados              24) Grenada               42) St Maarten (Dutch)

 7) Belize                25) Guadeloupe            43) St Martin (French)

 8) Bolivia               26) Guatemala             44) St Pierre & Miquelon

 9) Brazil                27) Guyana                45) St Vincent

10) Canada                28) Haiti                 46) Suriname

11) Caribbean NL          29) Honduras              47) Trinidad & Tobago

12) Cayman Islands        30) Jamaica               48) Turks & Caicos Is

13) Chile                 31) Martinique            49) United States

14) Colombia              32) Mexico                50) Uruguay

15) Costa Rica            33) Montserrat            51) Venezuela

16) Cuba                  34) Nicaragua             52) Virgin Islands (UK)

17) Curacao               35) Panama                53) Virgin Islands (US)

18) Dominica              36) Paraguay

#? 49

Please select one of the following time zone regions.

 1) Eastern (most areas)            16) Central - ND (Morton rural)

 2) Eastern - MI (most areas)       17) Central - ND (Mercer)

 3) Eastern - KY (Louisville area)  18) Mountain (most areas)

 4) Eastern - KY (Wayne)            19) Mountain - ID (south); OR (east)

 5) Eastern - IN (most areas)       20) MST - Arizona (except Navajo)

 6) Eastern - IN (Da, Du, K, Mn)    21) Pacific

 7) Eastern - IN (Pulaski)          22) Alaska (most areas)

 8) Eastern - IN (Crawford)         23) Alaska - Juneau area

 9) Eastern - IN (Pike)             24) Alaska - Sitka area

10) Eastern - IN (Switzerland)      25) Alaska - Annette Island

11) Central (most areas)            26) Alaska - Yakutat

12) Central - IN (Perry)            27) Alaska (west)

13) Central - IN (Starke)           28) Aleutian Islands

14) Central - MI (Wisconsin border) 29) Hawaii

15) Central - ND (Oliver)

#? 1


The following information has been given:


      United States

      Eastern (most areas)


Therefore TZ='America/New_York' will be used.

Local time is now:      Tue May 31 11:21:04 EDT 2016.

Universal Time is now:  Tue May 31 15:21:04 UTC 2016.

Is the above information OK?

1) Yes

2) No

#? 1


You can make this change permanent for yourself by appending the line

      TZ='America/New_York'; export TZ

to the file '.profile' in your home directory; then log out and log in again.


Here is that TZ value again, this time on standard output so that you

can use the /usr/bin/tzselect command in shell scripts:

America/New_York

[ehemdal@localhost ~]$

The file ~/.profile (if it exists) gets run when you log in and will set the TZ variable for you. You might use a different initialization file if you use a shell other than /bin/sh or /bin/bash. I set this and logged back in. If you update a file like /etc/profile, you can apply this for all users.

Here you can see that TZ is set for my user (ehemdal), but NOT for the root user.

TZ and the Time and Date Seen by Users –

What happens if a user is connected to your server from a different timezone? The TZ variable helps keep the user’s local time correct while keeping management of timezones and Daylight Saving Time in the hands of the operating system. This affects the time that Cache’ uses as well. For an example, I decided to change my timezone to that for Honolulu.

Here are two screenshots that show the result.

My user’s process has set TZ to Pacific/Honolulu. Root’s process does not have TZ set (so is using the system timezone America/New_York). At operating system level (with the date command), the display reflects local time for both users. The date command reflects the user’s local time (HST for user ehemdal and EDT for root). Since $HOROLOG obtains its value from the OS time available to the user process, the $H values are DIFFERENT for the two users.

I chose Honolulu time as an interesting example because Hawaii does not observe Daylight Saving Time. By setting TZ properly for all users, the local time can “spring ahead and fall back” for users who observe DST, and remain stable for users who don’t.