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country codes will be accepted.

=item B<country_name>

=item B<cn>

An English name for a country, possibly in a modified form, optimised
to help humans find the right entry in alphabetical lists.  This is not
necessarily identical to the country's standard short or long name.

=item B<region_description>

=item B<rd>

A brief English description of a segment of a country, used to distinguish
between the regions of a single country.  Empty string if the country
has only one region for timezone purposes.

=item B<canonical_zone_name>

=item B<k>

Hierarchical name of a canonical timezone, such as "B<Europe/London>".
On input, a timezone name must be supplied with exact casing, and only
names of defined canonical timezones will be accepted.

=item B<offset @> I<absolute-time>

=item B<o @> I<absolute-time>

The offset from UT that a timezone observes at the specified absolute
time.  See below for the format in which the absolute time must be stated.
An offset is expressed in hours, minutes, and seconds, two digits of each,
with a leading sign.  On output, components are separated by colons, and
trailing zero-valued components are omitted.  On input, colon separators
are optional, and trailing zero-valued components are optional.

=item B<initialism @> I<absolute-time>

=item B<i @> I<absolute-time>

The initialism used to refer to a timezone at the specified absolute time.
See below for the format in which the absolute time must be stated.
On input, an initialism must be supplied with exact casing.

=item B<dst_status @> I<absolute-time>

=item B<d @> I<absolute-time>

Whether a timezone is observing DST at the specified absolute time.
See below for the format in which the absolute time must be stated.
It is expressed as a single character: a "C<+>" indicates DST, and a
"C<->" indicates not DST.

=item B<local_time @> I<absolute-time>

=item B<t @> I<absolute-time>

The local time in a timezone at the specified absolute time.  See below
for the format in which the absolute time must be stated.  A local time
is expressed in the Gregorian calendar in ISO 8601 format: year, month,
day, hour, minute, and second.  The year is four digits, and the other
components two digits.  On output, date is separated from time of day by
a "B<T>", dash and colon separators are used between components, and all
components are emitted.  On input, the separators are optional, "B<T>" is
accepted case insensitively and can be surrounded or replaced with spaces,
and trailing components with the lowest possible value are optional.

=back

Where an absolute time must be used to parameterise an attribute, it
can be expressed in these forms:

=over

=item I<local-time> B<Z>

Time in UT expressed in ISO 8601 format.  The I<local-time> is as
described for the B<local_time> attribute, and "B<Z>" indicates that
it is UT.  "B<Z>" is accepted case insensitively and can be preceded
by spaces.

=item B<now>

The time at which the program is running.  A single time is used as "now"
for the entire program run, even if it takes a non-negligible amount of
time to run.

=back

=head2 Exceptions

Sometimes a proper value for an attribute is not available.  On output,
this doesn't prevent a line appearing, and the place of the attribute
is taken by a possibly-repeated punctuation character signalling the
kind of exception.  The kinds of exception are:

=over

=item B<~~~>

A particular local time doesn't exist because it is skipped as a timezone
changes offset.  Typically occurs at Spring DST changes.
(Currently this program doesn't support any queries that can generate
this exception.)

=item B<???>

The information in the Olson database is incomplete.  Typically occurs
for the distant future in timezones with tricky DST rules.

=item B<!!!>

There is definitively no applicable value.  Occurs where entities
don't match up at all, for example when requesting the area of a
non-geographical timezone.  Also occurs when requesting observance data
(such as offset) for a disused timezone (e.g., before the location
was inhabited).

=back