The tzdata packages contain data files with rules for various time zones around the world. You may think that these data files are static and do not change much from year to year, but in fact many changes to the data files are required each year for various reasons.
For example, here is a partial list of changes to time zones in Red Hat Exterprise Linux during the first part of 2013:
Product Enhancement Advisory – RHEA-2012:1101-2
* Daylight Saving Time will be interrupted during the holy month of Ramadan in
Morocco (that is July 20 – August 19, 2012 in the Gregorian Calendar). This
update incorporates the exception so that Daylight Saving Time is turned off and
the time setting returned back to the standard time during Ramadan.
Product Enhancement Advisory – RHEA-2012:1338-2
* Daylight saving time in Fiji will start at 2:00 a.m. on Sunday, 21st October
2012, and end at 3 am on Sunday, 20th January 2013.
* Tokelau was listed in an incorrect time zone for as long as the Zoneinfo
project was in existence. The actual zone was supposed to be GMT-11 hours before
Tokelau was moved to the other side of the International Date Line at the end of
year 2011. The local time in Tokelau is now GMT+13.
Product Enhancement Advisory – RHEA-2012:1488-2
* On October 24 2012, the Jordanian Cabinet rescinded a 2012-10-14
instruction to switch from daylight saving time (DST) to standard time on
2012-10-26. Instead, Jordan will remain on local DST (ITC +3) for the
2012-2013 Jordanian winter. (BZ#871993, BZ#871791, BZ#871994, BZ#871995)
* Cuba, which was scheduled to move back to standard time on 2012-11-12,
switched to standard time on 2012-11-04.
* In Brazil, the North Region state, Tocantins, will observe DST in
2012-2013. This is the first time Tocantins has observed DST since 2003. By
contrast, Bahia, a Northeast Region state, will not observe DST in
2012-2013. Like Tocantins, Bahia stopped observing DST in 2003. Bahia
re-introduced DST on October 16 2011. On October 17 2012, however, Bahia
Governor, Jaques Wagner, announced DST would not be observed in 2012,
citing public surveys showing most Bahia residents were opposed to it.
* Israel has new DST rules as of 2013. DST now starts at 02:00 on the
Friday before the last Sunday in March. DST now ends at 02:00 on the first
Sunday after October 1, unless this day is also the second day of (Rosh
Hashanah). In this case, DST ends a day later, at 02:00 on the first Monday
after October 2.
* The Palestinian territories, which were scheduled to move back to
standard time on 2012-09-28, switched to standard time on 2012-09-21.
* Although Western Samoa has observed DST for two consecutive seasons
(2010-2011 and 2011-2012), there is no official indication of DST
continuing according to a set pattern for the foreseeable future. On
2012-09-04, the Samoan Ministry of Commerce, Industry, and Labour announced
Samoa would observe DST from Sunday, 2012-09-30 until Sunday 2012-04-07.
Product Enhancement Advisory – RHEA-2013:0182-2
* On Nov 10, 2012, Libya changed to the time zone UTC+1. Therefore, starting
from the year 2013 Libya will be switching to daylight saving time on the last
Friday of March and back to the standard time on the last Friday of October. The
time zone setting and the daylight saving time settings for the Africa/Tripoli
time zone have been updated accordingly.