=head1 INTRODUCTION

Every date in RT's DB is stored in UTC format. This affects charts
grouped by time periods (Annually, Monthly, etc.), in that they are by
default shown in UTC. To produce charts that are in a specific timezone,
we have to use database-specific functions to convert between timezones;
unsurprisingly, each DB has very different requirements.

=head1 CONFIGURATION

This code is experimental; you can enable it using the
C<$ChartsTimezonesInDB> configuration option.

=head1 DATABASE SPECIFIC NOTES

=head2 mysql

The time adjustment cannot simply be converted using a numeric time
shift, as this shift value depends on the daylight saving time
properties of the time zone.

mysql since 4.1.3 supports named timezones, but you have to fill special
tables with up-to-date timezone data. On modern systems, this is usually
a simple case of:

    mysql_tzinfo_to_sql /usr/share/zoneinfo | mysql -u root mysql

mysql's doc recommends you restart server after running this; you can
read more about mysql's timezone support at
L<http://dev.mysql.com/doc/refman/5.0/en/time-zone-support.html>

=head2 PostgreSQL

PostgreSQL uses your operating system's functions to convert timezones.
Thus, you don't need to do anything in particular except to make sure
that the data in F</usr/share/zoneinfo> is up to date. On some systems
this may mean upgrading a system package.

=head3 Note for users of Pg 7.2 and older or users upgraded from those

You should be sure that timestamps in RT DB have no TZ set. The
TIMESTAMP column type in PostgreSQL prior to Pg 7.3 had timezone info by
default; this has been removed in more recent versions.  If your RT
database has this embedded timezone info, you will need to alter the
columns to remove them before enabling this feature.

=head2 Other databases

There is no implementation for Oracle or SQLite at current.

=head1 FOR DEVELOPERS

=head2 PostgreSQL

We use the timestamp type for all datetime fields. It either has
timezone info or not, since by default Pg 7.3 and above have no
timezone. Conversion is kinda tricky:

    timezone('Europe/Moscow', timezone('UTC', column_without_tz_info))
    timezone('to_tz', timezone('from_tz', column_without_tz_info))

This function flips the HAS_TZ flag on the argument, and moves the
timestamp to UTC. The first call makes no conversion, but flips the
HAS_TZ flag; the second call flips it back and does actual conversion.

For more information, See
L<http://www.postgresql.org/docs/7.4/static/functions-datetime.html#FUNCTIONS-DATETIME-ZONECONVERT>
and
L<http://www.postgresql.org/docs/7.4/static/datatype-datetime.html#DATATYPE-TIMEZONES>

=head2 mysql

Once timezone information is loaded into tables on the server,
we have all the same set of named timezones in the system
and DateTime (DateTime project has copy of the TZ data in a module).

CONVERT_TZ(TS, from, to) exists since mysql 4.1.3. Note that it takes a
timestamp, so it only supports limitted date range (usuall 1970-2038).

=head2 Oracle

Look at FROM_TZ function.

=head2 SQLite

Has no apparent timezone support.

=cut