{- | Module: Database.PostgreSQL.Simple.Time Copyright: (c) 2012-2015 Leon P Smith License: BSD3 Maintainer: Leon P Smith Stability: experimental This module provides time types that supports positive and negative infinity, as well as some functions for converting to and from strings. Also, this module also contains commentary regarding postgresql's timestamp types, timekeeping in general, and how it relates to postgresql-simple. You can read more about PostgreSQL's date and time types at , and the IANA timezone database at . Stack Overflow also has some excellent commentary on time, if it is a wiki page or a highly upvoted question and answer. If the answer regarding time has not received hundreds of upvotes, then the answer is almost invariably completely and painfully wrong, even if it's the chosen answer or the most highly upvoted answer to a question. PostgreSQL's @timestamp with time zone@ (hereafter, @timestamptz@) can be converted to Haskell's 'Data.Time.UTCTime' and 'Data.Time.ZonedTime' types, because their values represent an unambiguous point in time. PostgreSQL's @timestamp without time zone@ (hereafter, @timestamp@) can be converted to Haskell's 'Data.Time.LocalTime', because values of these types can only be disambiguated in a larger context. While this behavior may be superficially counterintuitive because the names might suggest otherwise, this behavior is correct. In fact, the \"timezone\" language in both the postgresql and haskell types would be better read as \"offset (from UTC)\", thus we have postgresql's \"timestamp with offset\" corresponding to Haskell's \"time with the offset \'zero\'\" and Haskell's \"time with an offset (that might be nonzero)\". Similarly, postgresql's \"timestamp without an offset\" corresponds to Haskell's \"local time (without an offset)\". It's important to distinguish between an offset, a standard time, and a time zone. An offset is simply a difference of a local time from UTC, such as @+00@, @-05@, or @+05:30@. A standard time specifies an offset (which may vary throughout the year, due to daylight savings) that a region follows, such as Universal Coordinated Time (UTC), Eastern Standard Time\/Eastern Daylight Time (EST\/EDT), or India Standard Time (IST). And a timezone, much like a standard time, is a function from timestamps to offsets. A timezone is different from a standard time because different regions inside a standard time are governed by different civil authorities and have different histories of time. Thus, an IANA timezone is any region of the world that has had the same history of time since @1970-01-01 00:00+00@. For example, as of today, both @America\/New_York@ and @America\/Indiana\/Indianapolis@ are on the EST\/EDT time standard, but Indiana used to be on Central Standard Time until 1942, and did not observe daylight savings time (EST only) until 2006. Thus, the choice between these two timezones still matters if you are dealing with timestamps prior to 2006, and could become relevant again if (most of) Indiana moves back to Central Time. (Of course, if the Central to Eastern switch was the only difference, then these two timezones would be the same in IANA's eyes, due to their cutoff date of 1970-01-01.) Getting back to the coding practicalities, PostgreSQL's @timestamptz@ type does not actually store an offset; rather, it uses the offset provided to calculate UTC, and stores the timestamp as UTC. If an offset is not provided, the given timestamp is assumed to be a local time for whatever the @timezone@ variable is set to, and the IANA TZ database is consulted to calculate an offset from UTC for the time in question. Note that some local timestamps are ambiguous, in which case PostgreSQL chooses one of the possible offsets. When retrieving a @timestamptz@, the backend looks at the @timezone@ connection variable and then consults the IANA TZ database to calculate an offset for the timestamp in the given timezone. Note that while some of the information contained in the IANA TZ database is a bit of a standardized fiction, the conversion from UTC time to a (local time, offset) pair is always unambiguous, and the result can always be unambiguously converted back to UTC. Thus, postgresql-simple can interpret this result as a 'Data.Time.ZonedTime', or use the offset to convert to 'Data.Time.UTCTime'. By contrast, the @timestamp@ type ignores any offsets provided to it, and never sends back an offset. Thus, postgresql-simple equates this with 'Data.Time.LocalTime', which has no concept of an offset. One can convert between @timestamptz@ and @timestamp@ using the @AT TIME ZONE@ operator, whose semantics also demonstrates that @timestamptz@ is 'Data.Time.UTCTime' whereas @timestamp@ is 'Data.Time.LocalTime'. The default default setting for the @timezone@ variable is @\'localtime\'@, which normally corresponds to the server's local timezone. However, this default can be modified on the server side for an entire cluster, or on a per-user or per-database basis. On the client side, the timezone can be set via the @PGTZ@ environment variable on the client, or be modified at any time for the duration of the entire connection, just a single transaction, or the execution context of a server-side function. With a few caveats, postgresql-simple is designed so that you can both send and receive timestamps with the server and get a correct result, no matter what the @timezone@ setting is. But it is important to understand the caveats: 1. The correctness of server-side computations can depend on the @timezone@ setting, for example when adding an @interval@ to a @timestamptz@, or when type casting between @timestamp@ and @timestamptz@. 2. The offset in a 'Data.Time.ZonedTime' result will depend on the @timezone@ setting, although the result will always represent the same instant in time regardless of the offset. 3. Sending a 'Data.Time.LocalTime' and interpreting it as a @timestamptz@ can be useful, as it will be converted to UTC via the tz database, but correctness will depend on the @timezone@ setting. You may prefer to use an explicit @AT TIME ZONE@ instead. Furthermore, although these points don't involve the @timezone@ setting, they are related to the last point above: 1. Sending a 'Data.Time.UTCTime' and interpreting it as a @timestamp@ can be useful. In practice, the most common context used to disambiguate @timestamp@ is that it represents UTC, and this coding technique will work as expected in this context. 2. Sending a 'Data.Time.ZonedTime' and interpreting it as a @timestamp@ is almost always the wrong thing to do, as the offset will be ignored and discarded. This is likely to lead to inconsistencies in the database, and may lead to partial data loss. Moreover, one should keep in mind that "ZonedTime" is misnamed, and one should not conflate an offset with a timezone. There are a few limitations and caveats that one might need to be aware of with the current implementation when dealing with older timestamps: For sufficiently old timestamps in almost all timezones, the IANA TZ database specifies offsets from UTC that is not an integral number of minutes. This corresponds to local mean time; that is, astronomical time in the city that defines the timezone. Different timezones moved away from local mean time to a standard time at different points in history, so \"sufficiently old\" depends on the timezone in question. Thus, when retrieving a @timestamptz@ postgresql will in some cases provide seconds in the offset. For example: @ $ psql psql (9.4.5) Type \"help\" for help. lpsmith=> SET timezone TO \'America/New_York\'; SET lpsmith=> VALUES (\'1883-11-18 16:59:59+00\'::timestamptz), (\'1883-11-18 17:00:00+00\'::timestamptz); column1 ------------------------------ 1883-11-18 12:03:57-04:56:02 1883-11-18 12:00:00-05 (2 rows) @ Both of these timestamps can be parsed as a 'Data.Time.UTCTime' type, however 'Data.Time.ZonedTime' will fail on the former timestamp. Because 'Data.Time.ZonedTime' assumes that offsets are an integer number of minutes, there isn't an particularly good solution here. PostgreSQL, like most software, uses the proleptic Gregorian calendar for its date calculations, extending the Gregorian calendar backwards in time before its introduction and pretending that the Julian calendar does not exist. For most purposes, the adoption of the Gregorian calendar ranges from @1582-10-15@ to @1923-03-01@, depending on location and sometimes even political allegiances within a single location. Timestamps BCE are not supported. For example, PostgreSQL will emit \"@0045-01-01 BC@\" for the first proleptic Gregorian day of the year the Roman Empire adopted the Julian Calendar, but postgresql-simple does not (yet?) have the ability to either parse or generate this syntax. Unfortunately this syntax isn't convenient to print or especially parse. Also, postgresql itself cannot parse or print dates before @4714-11-24 BC@, which is the Julian date on the proleptic Gregorian Calendar. Although postgresql's timestamp types are perfectly capable of representing timestamps nearly 300,000 years in the past, using this would require postgresql-simple and other client programs to support binary parameters and results. Dealing with years BCE is also complicated slightly by the fact that Haskell's time library has a year \"0000\", which is a convention often used by astronomers, while postgresql adopts the more historically accurate convention that there is no year zero, but rather \"1 BCE\" was immediately followed by \"1 CE\". -} module Database.PostgreSQL.Simple.Time ( Unbounded(..) , Date , UTCTimestamp , ZonedTimestamp , LocalTimestamp , parseDay , parseUTCTime , parseZonedTime , parseLocalTime , parseTimeOfDay , parseDate , parseUTCTimestamp , parseZonedTimestamp , parseLocalTimestamp , dayToBuilder , utcTimeToBuilder , zonedTimeToBuilder , localTimeToBuilder , timeOfDayToBuilder , timeZoneToBuilder , dateToBuilder , utcTimestampToBuilder , zonedTimestampToBuilder , localTimestampToBuilder , unboundedToBuilder , nominalDiffTimeToBuilder ) where import Database.PostgreSQL.Simple.Time.Implementation