It is the small data that matters the most1, but it is the small data that is holding everything together. The small slowly changing reference tables are the linchpins. Unfortunately, too often politics gets in the way as those small tables, maintained by humans, don't get the attention they deserve; or in other words their owners, if they exists - many of these little tables are orphans, make changes without understanding the potential consequences on downstream systems.
Here is a little story as it happened over the weekend.
A friend of mine flew on Saturday night from Vienna to Yerevan. The plane was scheduled to leave Austria at 10:15 PM and to arrive at 4:35 AM in Armenia. However, last weekend was the weekend when the summer time or daylight saving time (DST) ended in many European countries, including Austria, but not in Armenia.
Armenia used DST in 1981-1995 and 1997-2011. This may look a little odd, but Armenia is bar far not the only region that has changed its policy over the years. Russia decided in 2011 not to switch to winter time and keep DST all year round, only to switch to permanent winter time last weekend again.
Back to my friend, she arrives early Sunday morning at the airport in Yerivan, but her friend who promised to pick her up is not there. After a little while she takes a cab to her friends house, making it just in time before he was about to leave for the airport. What happened? Well, her friend relied on his mobile phone for his wake-up call. However, his mobile phone wasn't aware of the fact that Armenia didn't have DST anymore and set his clock back by one hour, allowing him more sleep. His phone might either be a little old or getting its signal from Russia, I don't know. Yet, this little story illustrates nicely how reliant we are nowadays on our interconnected devices having access to the correct reference data.
Imagine being in an intensive care unit relying on medial devices that have to work correctly together on time signals, when they are sourced from different companies, countries and running on different systems. Try to stay healthy, particular in late March and October, would be my advise.
So, how do systems/computers actually know which timezone to use?
Most computers will use the tz reference code and database maintained by a group of volunteers until 2011 and now by IANA. Still, how do they get informed that a region or state decided to change the rules? By keeping their ears on the ground!
The following extract from the Asia file of the tz database 2014b release is quite telling:
# Armenia # From Paul Eggert (2006-03-22): # Shanks & Pottenger have Yerevan switching to 3:00 (with Russian DST) # in spring 1991, then to 4:00 with no DST in fall 1995, then # readopting Russian DST in 1997. Go with Shanks & Pottenger, even # when they disagree with others. Edgar Der-Danieliantz # reported (1996-05-04) that Yerevan probably wouldn't use DST # in 1996, though it did use DST in 1995. IATA SSIM (1991/1998) reports # that Armenia switched from 3:00 to 4:00 in 1998 and observed DST # after 1991, but started switching at 3:00s in 1998. # From Arthur David Olson (2011-06-15): # While Russia abandoned DST in 2011, Armenia may choose to # follow Russia's "old" rules. # From Alexander Krivenyshev (2012-02-10): # According to News Armenia, on Feb 9, 2012, # http://newsarmenia.ru/society/20120209/42609695.html # # The Armenia National Assembly adopted final reading of Amendments to the # Law "On procedure of calculation time on the territory of the Republic of # Armenia" according to which Armenia [is] abolishing Daylight Saving Time. # or # (brief) # http://www.worldtimezone.com/dst_news/dst_news_armenia03.html # Zone NAME GMTOFF RULES FORMAT [UNTIL] Zone Asia/Yerevan 2:58:00 - LMT 1924 May 2 3:00 - YERT 1957 Mar # Yerevan Time 4:00 RussiaAsia YER%sT 1991 Mar 31 2:00s 3:00 1:00 YERST 1991 Sep 23 # independence 3:00 RussiaAsia AM%sT 1995 Sep 24 2:00s 4:00 - AMT 1997 4:00 RussiaAsia AM%sT 2012 Mar 25 2:00s 4:00 - AMT
Does this all sounds familiar to you and the challenges in your own organisation? Well, I gave a talk on Small & Big Data recently with a colleague of mine, should you be interested to find out more about this topic.
1. or Tiny Data in Rasmus' case↩