Leap second handling in SUSE Linux products

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Environment

SUSE Linux Enterprise Server 11
SUSE Linux Enterprise Server 10
SUSE Linux Enterprise Server 9
SUSE Linux Enterprise Desktop 11
SUSE Linux Enterprise Desktop 10

Situation

Do SUSE's Linux products handle leap seconds correctly?

Resolution

Use NTP

For environments where proper processing of leap seconds is a concern, it is strongly recommended that Linux systems be configured to synchronise time using NTP against one or more stable, reliable time servers. If NTP is not used, a Linux system needs to rely on the system hardware clock which may not be reliable enough, and proper processing of time sensitive data cannot be guaranteed.

Details of NTP configuration can be found in the product documentation as well as in other knowledge base articles.

Leap second handling with NTP

When NTP is configured properly, the NTP daemon will be informed by the NTP servers when a leap second is introduced; it will then in turn provide this information to the Linux kernel which will process it; the kernel will log a message "Clock: inserting leap second 23:59:60 UTC".

Leap second processing without NTP

For a system where NTP has not been configured, leap seconds can still be processed properly (within the limits imposed by the unreliability of hardware clocks as discussed above), provided the maintenance updates of the timezone package are followed.

The core timezone package provides the data files used in conversion between Unix internal time (seconds since a particular moment in 1970) and civil time (including daylight saving time where applicable). It takes care of the data files under /usr/share/zoneinfo as well as of /etc/localtime.

For SUSE Linux products which are under general support, this package is updated whenever significant changes occur, such as changes in daylight saving time start or end dates, and the introduction of leap seconds.

For SUSE Linux products which are under extended support, customers entitled to extended support can open a service request with SUSE Technical Services to request updates of this package when needed.

Additional Information

Best common practices relating to leap seconds

For systems whose time keeping needs are demanding enough that leap seconds are of interest, best practices are:

Ensure that time synchronisation using NTP against one or more reliable time sources is set up.
Ensure that maintenance updates for the timezone package are applied when they become available.

Status regarding the 2012 leap second (Last updated: 2012-03-09)

A leap second has been scheduled for June, 30th 2012.

For SUSE Linux Enterprise Server 10 Service Pack 4 and SUSE Linux Enterprise Desktop 10 Service Pack 4, the timezone package has been updated to version 2012b-0.0.5 to reflect this.

For SUSE Linux Enterprise Server 11 Service Pack 1 and SUSE Linux Enterprise Desktop 11 Service Pack 1, the timezone package has been updated to version 2012b-0.3.2 to reflect this. This update also applies to Service Pack 2 according to the SUSE Linux Enterprise 11 Maintenance Model.

Status regarding the 2008-2009 leap second (Last updated: 2008-11-14)

For 2008, there was a leap second at the end of the year.

For SUSE Linux Enterprise Server 10 Service Pack 1 and SUSE Linux Enterprise Desktop 10 Service Pack 1, the timezone package has been updated for this as of version 2.4-31.43.13.

For SUSE Linux Enterprise Server 10 Service Pack 2 and SUSE Linux Enterprise Desktop 10 Service Pack 2, the timezone package has been updated for this as of version 2.4-31.58.

For SUSE Linux Enterprise 9 Service Pack 4, the timezone package has been updated for this as of version 2.3.3-98.101.

Background: Leap second

Accurate timekeeping is of huge importance in science and in the economic systems of developing and developed nations. High precision time keeping is done using atomic clocks.
Leap seconds are a mechanism to adjust for differences between time as kept by atomic clocks and the notion of time based on solar days and years which is more intuitive, but less accurate due to astronomical phenomena such as the earth's rotation speed slowing down due to tidal friction.

Leap years are a a somewhat similar mechanism to deal with differences between time measured in solar days and time measured in solar years. But where leap years can be calculated algorithmically centuries in advance, the need for a leap second can only be accurately established through astronomical observations some months in advance.

The International Earth Rotation and Reference Systems Service (IERS) published bulletins to introduce leap seconds when needed.

More background information can be found in the Wikipedia article Leap Second.

Related time keeping topics

Daylight Saving Time (DST) is another topic of interest in time keeping. Refer to TID 3655154 - Daylight Saving Time (DST) Handling in Linux for details. This TID also provides information on some subsystems which do not use the data files from the timezone package for time keeping.

For more information about NTP, refer to the Wikipedia article Network Time Protocol and to the Network Time Protocol project website.

Disclaimer

This Support Knowledgebase provides a valuable tool for SUSE customers and parties interested in our products and solutions to acquire information, ideas and learn from one another. Materials are provided for informational, personal or non-commercial use within your organization and are presented "AS IS" WITHOUT WARRANTY OF ANY KIND.