Table 8-9. Date/Time Types

Note: The SQL standard requires that writing justtimestamp be equivalent totimestamp without time zone, andPostgreSQL honors that behavior. (Releases prior to 7.3 treated it astimestamp with time zone.)timestamptz is accepted as an abbreviation fortimestamp with time zone; this is aPostgreSQL extension.

Note: Whentimestamp values are stored as eight-byte integers (currently the default), microsecond precision is available over the full range of values. Whentimestamp values are stored as double precision floating-point numbers instead (a deprecated compile-time option), the effective limit of precision might be less than 6.timestamp values are stored as seconds before or after midnight 2000-01-01. Whentimestamp values are implemented using floating-point numbers, microsecond precision is achieved for dates within a few years of 2000-01-01, but the precision degrades for dates further away. Note that using floating-point datetimes allows a larger range oftimestamp values to be represented than shown above: from 4713 BC up to 5874897 AD.

The same compile-time option also determines whethertime andinterval values are stored as floating-point numbers or eight-byte integers. In the floating-point case, largeinterval values degrade in precision as the size of the interval increases.

8.5.1. Date/Time Input

Date and time input is accepted in almost any reasonable format, including ISO 8601,SQL-compatible, traditionalPOSTGRES, and others. For some formats, ordering of day, month, and year in date input is ambiguous and there is support for specifying the expected ordering of these fields. Set theDateStyle parameter toMDY to select month-day-year interpretation,DMY to select day-month-year interpretation, orYMD to select year-month-day interpretation.

PostgreSQL is more flexible in handling date/time input than theSQL standard requires. SeeAppendix B for the exact parsing rules of date/time input and for the recognized text fields including months, days of the week, and time zones.

Remember that any date or time literal input needs to be enclosed in single quotes, like text strings. Refer toSection 4.1.2.7 for more information.SQL requires the following syntax

type [ (p) ] 'value'

wherep is an optional precision specification giving the number of fractional digits in the seconds field. Precision can be specified fortime,timestamp, andinterval types. The allowed values are mentioned above. If no precision is specified in a constant specification, it defaults to the precision of the literal value.

1999-01-08 04:05:06

1999-01-08 04:05:06 -8:00

January 8 04:05:06 1999 PST

TIMESTAMP '2004-10-19 10:23:54'

TIMESTAMP '2004-10-19 10:23:54+02'

TIMESTAMP WITH TIME ZONE '2004-10-19 10:23:54+02'

8.5.2. Date/Time Output

The output format of the date/time types can be set to one of the four styles ISO 8601,SQL (Ingres), traditionalPOSTGRES (Unixdate format), or German. The default is theISO format. (TheSQL standard requires the use of the ISO 8601 format. The name of the"SQL" output format is a historical accident.)Table 8-14 shows examples of each output style. The output of thedate andtime types is of course only the date or time part in accordance with the given examples.

In theSQL and POSTGRES styles, day appears before month if DMY field ordering has been specified, otherwise month appears before day. (SeeSection 8.5.1 for how this setting also affects interpretation of input values.)Table 8-15 shows an example.

The date/time styles can be selected by the user using theSET datestyle command, theDateStyle parameter in thepostgresql.conf configuration file, or thePGDATESTYLE environment variable on the server or client. The formatting functionto_char (seeSection 9.8) is also available as a more flexible way to format date/time output.

8.5.3. Time Zones

Time zones, and time-zone conventions, are influenced by political decisions, not just earth geometry. Time zones around the world became somewhat standardized during the 1900s, but continue to be prone to arbitrary changes, particularly with respect to daylight-savings rules.PostgreSQL uses the widely-used IANA (Olson) time zone database for information about historical time zone rules. For times in the future, the assumption is that the latest known rules for a given time zone will continue to be observed indefinitely far into the future.

PostgreSQL endeavors to be compatible with theSQL standard definitions for typical usage. However, theSQL standard has an odd mix of date and time types and capabilities. Two obvious problems are:

• Although thedate type cannot have an associated time zone, thetime type can. Time zones in the real world have little meaning unless associated with a date as well as a time, since the offset can vary through the year with daylight-saving time boundaries.

• The default time zone is specified as a constant numeric offset fromUTC. It is therefore impossible to adapt to daylight-saving time when doing date/time arithmetic acrossDST boundaries.

To address these difficulties, we recommend using date/time types that contain both date and time when using time zones. We donot recommend using the typetime with time zone (though it is supported byPostgreSQL for legacy applications and for compliance with theSQL standard).PostgreSQL assumes your local time zone for any type containing only date or time.

All timezone-aware dates and times are stored internally inUTC. They are converted to local time in the zone specified by thetimezone configuration parameter before being displayed to the client.

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

• A full time zone name, for exampleAmerica/New_York. The recognized time zone names are listed in thepg_timezone_names view (seeSection 45.67).PostgreSQL uses the widely-used IANA time zone data for this purpose, so the same time zone names are also recognized by much other software.

• A time zone abbreviation, for examplePST. 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-date rules as well. The recognized abbreviations are listed in thepg_timezone_abbrevs view (seeSection 45.66). You cannot set the configuration parameterstimezone orlog_timezone to a time zone abbreviation, but you can use abbreviations in date/time input values and with theAT TIME ZONE operator.

• In addition to the timezone names and abbreviations,PostgreSQL will accept POSIX-style time zone specifications of the formSTDoffset orSTDoffsetDST, whereSTD is a zone abbreviation,offset is a numeric offset in hours west from UTC, andDST is an optional daylight-savings zone abbreviation, assumed to stand for one hour ahead of the given offset. For example, ifEST5EDT were not already a recognized zone name, it would be accepted and would be functionally equivalent to United States East Coast time. When a daylight-savings zone name is present, it is assumed to be used according to the same daylight-savings transition rules used in the IANA time zone database'sposixrules entry. In a standardPostgreSQL installation,posixrules is the same asUS/Eastern, so that POSIX-style time zone specifications follow USA daylight-savings rules. If needed, you can adjust this behavior by replacing theposixrules file.

In short, this is the difference between abbreviations and full names: abbreviations represent a specific offset from UTC, whereas many of the full names imply a local daylight-savings time rule, and so have two possible UTC offsets. As an example,2014-06-04 12:00 America/New_York represents noon local time in New York, which for this particular date was Eastern Daylight Time (UTC-4). So2014-06-04 12:00 EDT specifies that same time instant. But2014-06-04 12:00 EST specifies noon Eastern Standard Time (UTC-5), regardless of whether daylight savings was nominally in effect on that date.

To complicate matters, some jurisdictions have used the same timezone abbreviation to mean different UTC offsets at different times; for example, in MoscowMSK has meant UTC+3 in some years and UTC+4 in others.PostgreSQL interprets such abbreviations according to whatever they meant (or had most recently meant) on the specified date; but, as with theEST example above, this is not necessarily the same as local civil time on that date.

One should be wary that the POSIX-style time zone feature can lead to silently accepting bogus input, since there is no check on the reasonableness of the zone abbreviations. For example,SET TIMEZONE TO FOOBAR0 will work, leaving the system effectively using a rather peculiar abbreviation for UTC. Another issue to keep in mind is that in POSIX time zone names, positive offsets are used for locationswest of Greenwich. Everywhere else,PostgreSQL follows the ISO-8601 convention that positive timezone offsets areeast of Greenwich.

In all cases, timezone names and abbreviations are recognized case-insensitively. (This is a change fromPostgreSQL versions prior to 8.2, which were case-sensitive in some contexts but not others.)

Neither timezone names nor abbreviations are hard-wired into the server; they are obtained from configuration files stored under.../share/timezone/ and.../share/timezonesets/ of the installation directory (seeSection B.3).

Thetimezone configuration parameter can be set in the filepostgresql.conf, or in any of the other standard ways described inChapter 18. There are also several special ways to set it:

• Iftimezone is not specified inpostgresql.conf or as a server command-line option, the server attempts to use the value of theTZ environment variable as the default time zone. IfTZ is not defined or is not any of the time zone names known toPostgreSQL, the server attempts to determine the operating system's default time zone by checking the behavior of the C library functionlocaltime(). The default time zone is selected as the closest match amongPostgreSQL's known time zones. (These rules are also used to choose the default value oflog_timezone, if not specified.)

• TheSQL commandSET TIME ZONE sets the time zone for the session. This is an alternative spelling ofSET TIMEZONE TO with a more SQL-spec-compatible syntax.

• ThePGTZ environment variable is used bylibpq clients to send aSET TIME ZONE command to the server upon connection.

8.5.4. Interval Input

interval values can be written using the following verbose syntax:

[@]quantityunit [quantityunit...] [direction]

wherequantity is a number (possibly signed);unit ismicrosecond,millisecond,second,minute,hour,day,week,month,year,decade,century,millennium, or abbreviations or plurals of these units;direction can beago or empty. The at sign (@) is optional noise. The amounts of the different units are implicitly added with appropriate sign accounting.ago negates all the fields. This syntax is also used for interval output, ifIntervalStyle is set topostgres_verbose.

Quantities of days, hours, minutes, and seconds can be specified without explicit unit markings. For example,'1 12:59:10' is read the same as'1 day 12 hours 59 min 10 sec'. Also, a combination of years and months can be specified with a dash; for example'200-10' is read the same as'200 years 10 months'. (These shorter forms are in fact the only ones allowed by theSQL standard, and are used for output whenIntervalStyle is set tosql_standard.)

Interval values can also be written as ISO 8601 time intervals, using either the"format with designators" of the standard's section 4.4.3.2 or the"alternative format" of section 4.4.3.3. The format with designators looks like this:

Pquantityunit [quantityunit ...] [ T [quantityunit ...]]

The string must start with aP, and may include aT that introduces the time-of-day units. The available unit abbreviations are given inTable 8-16. Units may be omitted, and may be specified in any order, but units smaller than a day must appear afterT. In particular, the meaning ofM depends on whether it is before or afterT.

In the alternative format:

P [years-months-days] [ Thours:minutes:seconds]

the string must begin withP, and aT separates the date and time parts of the interval. The values are given as numbers similar to ISO 8601 dates.

When writing an interval constant with afields specification, or when assigning a string to an interval column that was defined with afields specification, the interpretation of unmarked quantities depends on thefields. For exampleINTERVAL '1' YEAR is read as 1 year, whereasINTERVAL '1' means 1 second. Also, field values"to the right" of the least significant field allowed by thefields specification are silently discarded. For example, writingINTERVAL '1 day 2:03:04' HOUR TO MINUTE results in dropping the seconds field, but not the day field.

According to theSQL standard all fields of an interval value must have the same sign, so a leading negative sign applies to all fields; for example the negative sign in the interval literal'-1 2:03:04' applies to both the days and hour/minute/second parts.PostgreSQL allows the fields to have different signs, and traditionally treats each field in the textual representation as independently signed, so that the hour/minute/second part is considered positive in this example. IfIntervalStyle is set tosql_standard then a leading sign is considered to apply to all fields (but only if no additional signs appear). Otherwise the traditionalPostgreSQL interpretation is used. To avoid ambiguity, it's recommended to attach an explicit sign to each field if any field is negative.

Internallyinterval values are stored as months, days, and seconds. This is done because the number of days in a month varies, and a day can have 23 or 25 hours if a daylight savings time adjustment is involved. The months and days fields are integers while the seconds field can store fractions. Because intervals are usually created from constant strings ortimestamp subtraction, this storage method works well in most cases. Functionsjustify_days andjustify_hours are available for adjusting days and hours that overflow their normal ranges.

In the verbose input format, and in some fields of the more compact input formats, field values can have fractional parts; for example'1.5 week' or'01:02:03.45'. Such input is converted to the appropriate number of months, days, and seconds for storage. When this would result in a fractional number of months or days, the fraction is added to the lower-order fields using the conversion factors 1 month = 30 days and 1 day = 24 hours. For example,'1.5 month' becomes 1 month and 15 days. Only seconds will ever be shown as fractional on output.

Table 8-17 shows some examples of validinterval input.

8.5.5. Interval Output

The output format of the interval type can be set to one of the four stylessql_standard,postgres,postgres_verbose, oriso_8601, using the commandSET intervalstyle. The default is thepostgres format.Table 8-18 shows examples of each output style.

Thesql_standard style produces output that conforms to the SQL standard's specification for interval literal strings, if the interval value meets the standard's restrictions (either year-month only or day-time only, with no mixing of positive and negative components). Otherwise the output looks like a standard year-month literal string followed by a day-time literal string, with explicit signs added to disambiguate mixed-sign intervals.

The output of thepostgres style matches the output ofPostgreSQL releases prior to 8.4 when theDateStyle parameter was set toISO.

The output of thepostgres_verbose style matches the output ofPostgreSQL releases prior to 8.4 when theDateStyle parameter was set to non-ISO output.

The output of theiso_8601 style matches the"format with designators" described in section 4.4.3.2 of the ISO 8601 standard.

8.5.6. Internals

PostgreSQL uses Julian dates for all date/time calculations. This has the useful property of correctly calculating dates from 4713 BC to far into the future, using the assumption that the length of the year is 365.2425 days.

Date conventions before the 19th century make for interesting reading, but are not consistent enough to warrant coding into a date/time handler.