After a long discussion of the variation in European timezones seismo!mcvax!cwi.nl!dik (dik t. winter, cwi, amsterdam, nederland) states:
So what is my conclusion? It is nice to automatically switch to and from DST, however it must be easily possible to override the rules. (In some countries in Europe during the period 1977-1982 it was not known until a few days in advance whether a switch would occur or not, and what day it would occur!)
The problem occurs when the rules are embedded in a library routine or a difficult to change file. If the DST rules for the local time zone are kept in a file which can be easily and quickly edited then this can be done very quickly when necessary by those who should know (people living in the local time zone). Rather than a file of rules I prefer a file of change dates along with information about how much time is to be added or subtracted and the appropriate time zone name. The format should allow for partial hours, negative savings ("daylight spending time"?) and double daylight time. Arthur Olsen's timezone compiler generates a file of the proper type. However, I am not sure it provides the necessary flexibility for quick local changes of all types.