Clarissa Gammelt wrote on 1998-04-07:
How often are there changes to the time difference information (approximately)?
A couple of times per year, whenever Arthur David Olson has time, received patches, or feels that it is necessary.
Who provides that information?
There are many sources, including the International Air Transport Association <http://www.iata.org/>, international phone operators, media reports, specialized books, and many personal investigations of the tz mailing list members.
When in general are the dates for those changes? Are they regular, or each year different? Are there any rules?
No rules, because the politicians who mess around with time zone legislation also follow no rules.
Which countries do or don't have summer or winter time?
Changes every year. Check the data base. Markus -- Markus G. Kuhn, Security Group, Computer Lab, Cambridge University, UK email: mkuhn at acm.org, home page: <http://www.cl.cam.ac.uk/~mgk25/>