    {- |
    Module:      Database.PostgreSQL.Simple.Time
    Copyright:   (c) 2012-2015 Leon P Smith
    License:     BSD3
    Maintainer:  Leon P Smith <leon@melding-monads.com>
    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,  civil timekeeping in general,  and how it relates to
    postgresql-simple. You can read more about PostgreSQL's date and time types
    at <https://www.postgresql.org/docs/9.5/static/datatype-datetime.html>,
    and the IANA time zone database at <https://en.wikipedia.org/wiki/Tz_database>.

    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 about a hundred upvotes at least,  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 values of these types represent a self-contained, 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 are ambiguous by themselves,  and require context to disambiguate.

    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 time zone, much like a standard time, is a function from
    timestamps to offsets.

    A time zone is different from a standard time because different regions
    inside a standard time can be governed by different civil authorities with
    different laws and thus have different histories of civil time.  An IANA
    time zone is any region of the world that has had the same history of
    civil 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 time zones 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 time zones would be the same in
    IANA's eyes,  due to their cutoff date of 1970-01-01.)

    Getting back to 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 while most (local timestamp, time zone) pairs correspond to exactly
    one UTC timestamp, some correspond to two UTC timestamps,  while others
    correspond to none at all.   The ambiguous case occurs when the civil time
    is rolled back,  making a calendar day longer than 24 hours.  In this case,
    PostgreSQL silently chooses the second, later possibility.  The inconsistent
    case occurs when the civil time is moved forward,  making a calendar day less
    than 24 hours.  In this case,  PostgreSQL silently assumes the local time
    was read off a clock that had not been moved forward at the prescribed time,
    and moves the clock forward for you.   Thus,  converting from local time
    to UTC need not be monotonic,  if these inconsistent cases are allowed.

    When retrieving a @timestamptz@,  the backend looks at the @time zone@
    connection variable and then consults the IANA TZ database to calculate
    an offset for the timestamp in the given time zone.

    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 in a particular time zone is always unambiguous,
    and the result can always be unambiguously converted back to UTC.  Thus,
    postgresql-simple can interpret such a result as a 'Data.Time.ZonedTime',
    or use the offset to convert back 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'.

    PostgreSQL's @timezone@ is a per-connection variable that by default is
    initialized to @\'localtime\'@,  which normally corresponds to the server's
    time zone.  However, this default can be modified on the server side for an
    entire cluster, or on a per-user or per-database basis.  Moreover, a client
    can modify their instance of the variable at any time,  and can apply that
    change to the remaining duration of the connection, the current transaction,
    or the execution context of a server-side function.  In addition, upon
    connection initialization, the libpq client checks for the existence of
    the @PGTZ@  environment variable, and if it exists, modifies @timezone@
    accordingly.

    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.  Examples include adding an @interval@ to a @timestamptz@, or
       type casting between @timestamp@ and @timestamptz@,  or applying
       the @DATE@ function to a @timestamptz@.

    2. The (localtime, offset) pair contained 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 time zone.

    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@ conversion instead, which would avoid
       this contextual dependence.

    Furthermore,  although these following 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 situation.

    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.

    When dealing with local timestamps that refer to the future,  it is often
    useful to store it as a local time in a @timestamp@ column and store the
    time zone in a second column.  One reason to do this is so that you can
    convert to UTC on the fly as needed, and be protected against future changes
    to the TZ database due to changes in local time standards.  In any case,
    'Data.Time.ZonedTime' is not suitable for this application, because despite
    its name,  it represents an offset and not a time zone.  Time zones can change;
    offsets do not.  In reality,  we can't convert a local timestamp that occurs
    sufficiently far in the future to UTC, because we don't know how to do it yet.

    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 time zones,  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 time zone.  Different time zones moved
    away from local mean time to a standard time at different points in
    history,  so \"sufficiently old\" depends on the time zone 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
         , parseCalendarDiffTime
         , dayToBuilder
         , utcTimeToBuilder
         , zonedTimeToBuilder
         , localTimeToBuilder
         , timeOfDayToBuilder
         , timeZoneToBuilder
         , dateToBuilder
         , utcTimestampToBuilder
         , zonedTimestampToBuilder
         , localTimestampToBuilder
         , unboundedToBuilder
         , nominalDiffTimeToBuilder
         , calendarDiffTimeToBuilder
         ) where

    import Database.PostgreSQL.Simple.Time.Implementation