Additional source to guess the local timezone ID on linux

David Holmes david.holmes at
Mon Aug 26 01:59:38 UTC 2013

On 24/08/2013 7:09 AM, Omair Majid wrote:
> Hi,
> The algorithm that OpenJDK uses to guess the local timezone ID on Linux
> goes like this:
> 1. If TZ environment variable is set, use that
> 2. If /etc/timezone is readable, read the time zone from there
> 3. If /etc/localtime is a symlink, resolve that, and use the name to
> guess the time zone.
> 4. Scan /usr/share/zoneinfo for a file whose contents match the contents
> of /etc/localtime.
> Step 4 (if it is ever reached) is probably going to lead to incorrect
> results since there are a number of timezones that have the same
> zoneinfo data (such as Europe/London and Europe/Belfast). So it seems
> sensible to me to try and use additional sources to guess the timezone
> ID before resorting to file content comparisons.
> The webrev adds a step between 2 and 3 that reads and parses
> /etc/sysconfig/clock to extract the timezone:
> This file exists on some Red Hat Enterprise Linux (and derivative)
> distributions and contains contents that look this:
>> # The time zone of the system is defined by the contents of /etc/localtime.
>> # This file is only for evaluation by system-config-date, do not rely on its
>> # contents elsewhere.
>> ZONE="Europe/Zurich"

Surely the implication here is that if this file exists and has this 
data then /etc/localtime should be correctly configured - hence we 
should use that. ie at best this should come after step 3 not before it. 
But even then when someone writes "do not rely on its
contents elsewhere" I'm inclined to do what they suggest!


> With this, we should be able to identify the exact timezone ID.
> Thanks,
> Omair

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