Table 8-9. Date/Time Types
|Name||Storage Size||Description||Low Value||High Value||Resolution|
|timestamp [ (p) ] [ without time zone ]||8 bytes||both date and time||4713 BC||5874897 AD||1 microsecond / 14 digits|
|timestamp [ (p) ] with time zone||8 bytes||both date and time, with time zone||4713 BC||5874897 AD||1 microsecond / 14 digits|
|interval [ (p) ]||12 bytes||time intervals||-178000000 years||178000000 years||1 microsecond / 14 digits|
|date||4 bytes||dates only||4713 BC||5874897 AD||1 day|
|time [ (p) ] [ without time zone ]||8 bytes||times of day only||00:00:00||24:00:00||1 microsecond / 14 digits|
|time [ (p) ] with time zone||12 bytes||times of day only, with time zone||00:00:00+1459||24:00:00-1459||1 microsecond / 14 digits|
Note: Prior to PostgreSQL 7.3, writing just timestamp was equivalent to timestamp with time zone. This was changed for SQL compliance.
time, timestamp, and interval accept an optional precision value p which specifies the number of fractional digits retained in the seconds field. By default, there is no explicit bound on precision. The allowed range of p is from 0 to 6 for the timestamp and interval types.
Note: When timestamp values are stored as double precision floating-point numbers (currently the default), the effective limit of precision may be less than 6. timestamp values are stored as seconds before or after midnight 2000-01-01. Microsecond precision is achieved for dates within a few years of 2000-01-01, but the precision degrades for dates further away. When timestamp values are stored as eight-byte integers (a compile-time option), microsecond precision is available over the full range of values. However eight-byte integer timestamps have a more limited range of dates than shown above: from 4713 BC up to 294276 AD. The same compile-time option also determines whether time and interval values are stored as floating-point or eight-byte integers. In the floating-point case, large interval values degrade in precision as the size of the interval increases.
For the time types, the allowed range of p is from 0 to 6 when eight-byte integer storage is used, or from 0 to 10 when floating-point storage is used.
The type time with time zone is defined by the SQL standard, but the definition exhibits properties which lead to questionable usefulness. In most cases, a combination of date, time, timestamp without time zone, and timestamp with time zone should provide a complete range of date/time functionality required by any application.
The types abstime and reltime are lower precision types which are used internally. You are discouraged from using these types in new applications and are encouraged to move any old ones over when appropriate. Any or all of these internal types might disappear in a future release.
Date and time input is accepted in almost any reasonable format, including ISO 8601, SQL-compatible, traditional POSTGRES, and others. For some formats, ordering of month, day, and year in date input is ambiguous and there is support for specifying the expected ordering of these fields. Set the DateStyle parameter to MDY to select month-day-year interpretation, DMY to select day-month-year interpretation, or YMD to select year-month-day interpretation.
PostgreSQL is more flexible in handling date/time input than the SQL standard requires. See Appendix 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 to Section 188.8.131.52 for more information. SQL requires the following syntax
type [ (p) ] 'value'
where p in the optional precision specification is an integer corresponding to the number of fractional digits in the seconds field. Precision can be specified for time, timestamp, and interval 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.
Table 8-10 shows some possible inputs for the date type.
Table 8-10. Date Input
|January 8, 1999||unambiguous in any datestyle input mode|
|1999-01-08||ISO 8601; January 8 in any mode (recommended format)|
|1/8/1999||January 8 in MDY mode; August 1 in DMY mode|
|1/18/1999||January 18 in MDY mode; rejected in other modes|
|01/02/03||January 2, 2003 in MDY mode; February 1, 2003 in DMY mode; February 3, 2001 in YMD mode|
|1999-Jan-08||January 8 in any mode|
|Jan-08-1999||January 8 in any mode|
|08-Jan-1999||January 8 in any mode|
|99-Jan-08||January 8 in YMD mode, else error|
|08-Jan-99||January 8, except error in YMD mode|
|Jan-08-99||January 8, except error in YMD mode|
|19990108||ISO 8601; January 8, 1999 in any mode|
|990108||ISO 8601; January 8, 1999 in any mode|
|1999.008||year and day of year|
|January 8, 99 BC||year 99 before the Common Era|
The time-of-day types are time [ (p) ] without time zone and time [ (p) ] with time zone. Writing just time is equivalent to time without time zone.
Valid input for these types consists of a time of day followed by an optional time zone. (See Table 8-11 and Table 8-12.) If a time zone is specified in the input for time without time zone, it is silently ignored. You can also specify a date but it will be ignored, except when you use a time zone name that involves a daylight-savings rule, such as America/New_York. In this case specifying the date is required in order to determine whether standard or daylight-savings time applies. The appropriate time zone offset is recorded in the time with time zone value.
Table 8-11. Time Input
|04:05 AM||same as 04:05; AM does not affect value|
|04:05 PM||same as 16:05; input hour must be <= 12|
|04:05:06 PST||time zone specified by abbreviation|
|2003-04-12 04:05:06 America/New_York||time zone specified by full name|
Table 8-12. Time Zone Input
|PST||Abbreviation (for Pacific Standard Time)|
|America/New_York||Full time zone name|
|PST8PDT||POSIX-style time zone specification|
|-8:00||ISO-8601 offset for PST|
|-800||ISO-8601 offset for PST|
|-8||ISO-8601 offset for PST|
|zulu||Military abbreviation for UTC|
|z||Short form of zulu|
Refer to Section 8.5.3 for more information on how to specify time zones.
Valid input for the time stamp types consists of a concatenation of a date and a time, followed by an optional time zone, followed by an optional AD or BC. (Alternatively, AD/BC can appear before the time zone, but this is not the preferred ordering.) Thus
1999-01-08 04:05:06 -8:00
are valid values, which follow the ISO 8601 standard. In addition, the wide-spread format
January 8 04:05:06 1999 PST
The SQL standard differentiates timestamp without time zone and timestamp with time zone literals by the presence of a "+" or "-". Hence, according to the standard,
TIMESTAMP '2004-10-19 10:23:54'
is a timestamp without time zone, while
TIMESTAMP '2004-10-19 10:23:54+02'
is a timestamp with time zone. PostgreSQL never examines the content of a literal string before determining its type, and therefore will treat both of the above as timestamp without time zone. To ensure that a literal is treated as timestamp with time zone, give it the correct explicit type:
TIMESTAMP WITH TIME ZONE '2004-10-19 10:23:54+02'
In a literal that has been decided to be timestamp without time zone, PostgreSQL will silently ignore any time zone indication. That is, the resulting value is derived from the date/time fields in the input value, and is not adjusted for time zone.
For timestamp with time zone, the internally stored value is always in UTC (Universal Coordinated Time, traditionally known as Greenwich Mean Time, GMT). An input value that has an explicit time zone specified is converted to UTC using the appropriate offset for that time zone. If no time zone is stated in the input string, then it is assumed to be in the time zone indicated by the system's timezone parameter, and is converted to UTC using the offset for the timezone zone.
When a timestamp with time zone value is output, it is always converted from UTC to the current timezone zone, and displayed as local time in that zone. To see the time in another time zone, either change timezone or use the AT TIME ZONE construct (see Section 9.9.3).
Conversions between timestamp without time zone and timestamp with time zone normally assume that the timestamp without time zone value should be taken or given as timezone local time. A different zone reference can be specified for the conversion using AT TIME ZONE.
interval values can be written with the following syntax:
[@] quantity unit [quantity unit...] [direction]
Where: quantity is a number (possibly signed); unit is microsecond, millisecond, second, minute, hour, day, week, month, year, decade, century, millennium, or abbreviations or plurals of these units; direction can be ago or empty. The at sign (@) is optional noise. The amounts of different units are implicitly added up with appropriate sign accounting.
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'.
The optional subsecond precision p should be between 0 and 6, and defaults to the precision of the input literal.
Internally interval 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.
Because intervals are usually created from constant strings
or timestamp subtraction, this storage
method works well in most cases. Functions
justify_hours are available for adjusting
days and hours that overflow their normal periods.
PostgreSQL supports several special date/time input values for convenience, as shown in Table 8-13. The values infinity and -infinity are specially represented inside the system and will be displayed the same way; but the others are simply notational shorthands that will be converted to ordinary date/time values when read. (In particular, now and related strings are converted to a specific time value as soon as they are read.) All of these values need to be written in single quotes when used as constants in SQL commands.
Table 8-13. Special Date/Time Inputs
|Input String||Valid Types||Description|
|epoch||date, timestamp||1970-01-01 00:00:00+00 (Unix system time zero)|
|infinity||timestamp||later than all other time stamps|
|-infinity||timestamp||earlier than all other time stamps|
|now||date, time, timestamp||current transaction's start time|
|today||date, timestamp||midnight today|
|tomorrow||date, timestamp||midnight tomorrow|
|yesterday||date, timestamp||midnight yesterday|
The following SQL-compatible functions can also be used to obtain the current time value for the corresponding data type: CURRENT_DATE, CURRENT_TIME, CURRENT_TIMESTAMP, LOCALTIME, LOCALTIMESTAMP. The latter four accept an optional subsecond precision specification. (See Section 9.9.4.) Note however that these are SQL functions and are not recognized as data input strings.
The output format of the date/time types can be set to one of the four styles ISO 8601, SQL (Ingres), traditional POSTGRES, and German, using the command SET datestyle. The default is the ISO format. (The SQL 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 the date and time types is of course only the date or time part in accordance with the given examples.
Table 8-14. Date/Time Output Styles
|ISO||ISO 8601/SQL standard||1997-12-17 07:37:16-08|
|SQL||traditional style||12/17/1997 07:37:16.00 PST|
|POSTGRES||original style||Wed Dec 17 07:37:16 1997 PST|
|German||regional style||17.12.1997 07:37:16.00 PST|
In the SQL and POSTGRES styles, day appears before month if DMY field ordering has been specified, otherwise month appears before day. (See Section 8.5.1 for how this setting also affects interpretation of input values.) Table 8-15 shows an example.
Table 8-15. Date Order Conventions
|datestyle Setting||Input Ordering||Example Output|
|SQL, DMY||day/month/year||17/12/1997 15:37:16.00 CET|
|SQL, MDY||month/day/year||12/17/1997 07:37:16.00 PST|
|Postgres, DMY||day/month/year||Wed 17 Dec 07:37:16 1997 PST|
interval output looks like the input format, except that units like century or week are converted to years and days and ago is converted to an appropriate sign. In ISO mode the output looks like
[ quantity unit [ ... ] ] [ days ] [ hours:minutes:seconds ]
The date/time styles can be selected by the user using the
SET datestyle command, the DateStyle
parameter in the postgresql.conf
configuration file, or the PGDATESTYLE
environment variable on the server or client. The formatting
to_char (see Section 9.8) is also available
as a more flexible way to format the date/time output.
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 1900's, but continue to be prone to arbitrary changes, particularly with respect to daylight-savings rules. PostgreSQL currently supports daylight-savings rules over the time period 1902 through 2038 (corresponding to the full range of conventional Unix system time). Times outside that range are taken to be in "standard time" for the selected time zone, no matter what part of the year they fall in.
PostgreSQL endeavors to be compatible with the SQL standard definitions for typical usage. However, the SQL standard has an odd mix of date and time types and capabilities. Two obvious problems are:
Although the date type does not have an associated time zone, the time type can. Time zones in the real world have little meaning unless associated with a date as well as a time, since the offset may vary through the year with daylight-saving time boundaries.
The default time zone is specified as a constant numeric offset from UTC. It is therefore not possible to adapt to daylight-saving time when doing date/time arithmetic across DST boundaries.
To address these difficulties, we recommend using date/time types that contain both date and time when using time zones. We recommend not using the type time with time zone (though it is supported by PostgreSQL for legacy applications and for compliance with the SQL standard). PostgreSQL assumes your local time zone for any type containing only date or time.
All timezone-aware dates and times are stored internally in UTC. They are converted to local time in the zone specified by the timezone 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 example America/New_York. The recognized time zone names are listed in the pg_timezone_names view (see Section 43.49). PostgreSQL uses the widely-used zic time zone data for this purpose, so the same names are also recognized by much 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 may imply a set of daylight savings transition-date rules as well. The recognized abbreviations are listed in the pg_timezone_abbrevs view (see Section 43.48). You cannot set the configuration parameter timezone using 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 of the form STDoffset or STDoffsetDST, where STD is a zone abbreviation, offset is a numeric offset in hours west from UTC, and DST is an optional daylight-savings zone abbreviation, assumed to stand for one hour ahead of the given offset. For example, if EST5EDT were not already a recognized zone name, it would be accepted and would be functionally equivalent to USA 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 zic time zone database's posixrules entry. In a standard PostgreSQL installation, posixrules is the same as US/Eastern, so that POSIX-style time zone specifications follow USA daylight-savings rules. If needed, you can adjust this behavior by replacing the posixrules file.
There is a conceptual and practical difference between the abbreviations and the full names: abbreviations always represent a fixed offset from UTC, whereas most of the full names imply a local daylight-savings time rule and so have two possible UTC offsets.
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.
In all cases, timezone names are recognized case-insensitively. (This is a change from PostgreSQL versions prior to 8.2, which were case-sensitive in some contexts and not others.)
Neither full 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 (see Section B.3).
If timezone is not specified in postgresql.conf nor as a server command-line option, the server attempts to use the value of the TZ environment variable as the default time zone. If TZ is not defined or is not any of the time zone names known to PostgreSQL, the server attempts to determine the operating system's default time zone by checking the behavior of the C library function localtime(). The default time zone is selected as the closest match among PostgreSQL's known time zones.
The SQL command SET TIME ZONE sets the time zone for the session. This is an alternative spelling of SET TIMEZONE TO with a more SQL-spec-compatible syntax.
The PGTZ environment variable, if set at the client, is used by libpq applications to send a SET TIME ZONE command to the server upon connection.
PostgreSQL uses Julian dates for all date/time calculations. They have the nice property of correctly predicting/calculating any date more recent than 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.