[tz] Irish Standard Time vs Irish Summer Time

[Paul Eggert]

Lester Caine wrote:
> so should there be any timezones other than a single
> European time in the tzdb database?

Yes, of course. The EU rules cover multiple time zones and the tzdata format 
needs different Zones for each. But it's more than that: the EU didn't 
standardize daylight-saving transition rules until 1977. So time zone histories 
that differ in the 1970-1976 period require different Zones regardless of EU 
rules after 1977.