What is an appropriate data type to store a timezone?
I'm thinking of simply using a string in the format "+hh:mm" (or "-hh:mm"). Is this both necessary and sufficient?
Note: I don't need to store the date or the time, just the timezone.
Answer
Unfortunately PostgreSQL doesn't offer a time zone data type, so you should probably use text.
interval seems like a logical option at first glance, and it is appropriate for some uses. However, it fails to consider daylight savings time, nor does it consider the fact that different regions in the same UTC offset have different DST rules.
There is not a 1:1 mapping from UTC offset back to time zone.
For example, the time zone for Australia/Sydney (New South Wales) is UTC+10 (EST), or UTC+11 (EDT) during daylight savings time. Yes, that's the same acronym EST that the USA uses; time zone acronyms are non-unique in the tzdata database, which is why Pg has the timezone_abbreviations setting. Worse, Brisbane (Queensland) is at almost the same longditude and is in UTC+10 EST ... but doesn't have daylight savings, so sometime it's at a -1 offset to New South Wales during NSW's DST.
(Update: More recently Australia adopted an A prefix, so it uses AEST as its eastern states TZ acronym, but EST and WST remain in common use).
Confusing much?
If all you need to store is a UTC offset then an interval is appropriate. If you want to store a time zone, store it as text. It's a pain to validate and to convert to a time zone offset at the moment, but at least it copes with DST.