[tz] changes to the TZ database over versions

Joris Van den Bogaert jorisvdb at esus.com
Sat May 31 22:36:43 UTC 2014


I'm investigating how to deal with timezones for an application that will get to
processes billions of timestamped events, generated in just about any part of the world.
Timestamps are in local time (without timezone, only post- and other codes) and we need 
them in UTC time.  Money is involved, so it is of critical importance that intervals 
between events are calculated correctly.

There's two steps involved in this:

- given some geo data and some lookup tables from a database like geopostcodes.com, determine the timezone id
- convert to UTC using the latest Time Zone Database

Development wise it's fairly simple to automate.  We're using joda time and this library allows to 
plugin a custom Timezone provider which can then use the latest TZ database.

However, I see some issues:

1) I noticed that certain timezone ids are deleted in newer versions: tzdata2014a contains 
America/Shiprock, newer versions don't.  This may lead to issues whenever recalculations are needed 
in the future.  What is the likelyhood that ids are deleted?

2) Do rules from the past ever change?  In other words, can we assume that a recalculation of a past 
local date to a UTC date will always yield the same result with newer versions of the TZ database?  
If so, is this a common thing and would one then recommend saving the TZ database version with 
which the conversion was performed along with the UTC date?

Hope one of you can shed some light!
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