Time zone: support fixed offset e.g. UTC+8

[jon-chuang]

Currently, we only support geographical timezones for interpretation of timezone strings in AT TIME ZONE and session time zone

Note that PG supports such strings.

[jon-chuang]

Seems that postgres' implementation is quite weird. asia/singapore is equivalent to utc-8, not utc+8... 🤔 , in fact set timezone = '+8' is UTC+8 as expected, but anything (literally any string) followed by +8 becomes UTC-8 🤔

[xiangjinwu]

https://www.postgresql.org/docs/current/datatype-datetime.html#DATATYPE-TIMEZONES

PostgreSQL allows you to specify time zones in three different forms:

• A full time zone name, for example America/New_York.
  • PostgreSQL uses the widely-used IANA time zone data for this purpose, so the same time zone names are also recognized by other software.
• A time zone abbreviation, for example PST. Such a specification merely defines a particular offset from UTC, in contrast to full time zone names which can imply a set of daylight savings transition rules as well.
  • You cannot set the configuration parameters TimeZone or log_timezone to a time zone abbreviation, but you can use abbreviations in date/time input values and with the AT TIME ZONE operator.
• In addition to the timezone names and abbreviations, PostgreSQL will accept POSIX-style time zone specifications, as described in Section B.5. This option is not normally preferable to using a named time zone, but it may be necessary if no suitable IANA time zone entry is available.
  • The positive sign is used for zones west of Greenwich. (Note that this is the opposite of the ISO-8601 sign convention used elsewhere in PostgreSQL.)

Let's stick to the first option.