On Mon, 21 Sep 1998, Ulrich Hausmann wrote:

> Remains open only my curiousity: how does NetBSD know about political
> agreements on summer time periods? :))

The zoneinfo files are created with a program called 'zic', which is part
of the standard distribution. Here's an excerpt from one of the zic source
files, /usr/src/share/zoneinfo/europe:

# Continental Europe

# EU rules are for the European Union, previously known as the EC, EEC,
# Common Market, etc.

# Rule  NAME    FROM    TO      TYPE    IN      ON      AT      SAVE
LETTER/S
Rule    EU      1977    1980    -       Apr     Sun>=1   1:00u  1:00    S
Rule    EU      1977    only    -       Sep     lastSun  1:00u  0       -
Rule    EU      1978    only    -       Oct      1       1:00u  0       -
Rule    EU      1979    1995    -       Sep     lastSun  1:00u  0       -
Rule    EU      1981    max     -       Mar     lastSun  1:00u  1:00    S
Rule    EU      1996    max     -       Oct     lastSun  1:00u  0       -

# W-Eur differs from EU only in that W-Eur uses standard time.
Rule    W-Eur   1977    1980    -       Apr     Sun>=1   1:00s  1:00    S
Rule    W-Eur   1977    only    -       Sep     lastSun  1:00s  0       -
Rule    W-Eur   1978    only    -       Oct      1       1:00s  0       -
Rule    W-Eur   1979    1995    -       Sep     lastSun  1:00s  0       -
Rule    W-Eur   1981    max     -       Mar     lastSun  1:00s  1:00    S
Rule    W-Eur   1996    max     -       Oct     lastSun  1:00s  0       -

# Older C-Eur rules are for convenience in the tables.
# From 1977 on, C-Eur differs from EU only in that C-Eur uses standard
time.
Rule    C-Eur   1916    only    -       Apr     30      23:00   1:00    S
Rule    C-Eur   1916    only    -       Oct      1       1:00   0       -
Rule    C-Eur   1917    1918    -       Apr     Mon>=15  2:00s  1:00    S
Rule    C-Eur   1917    1918    -       Sep     Mon>=15  2:00s  0       -
Rule    C-Eur   1940    only    -       Apr      1       2:00s  1:00    S

That file also has detailed comments, naming the law(s) that lead to many
of the changes.