Date and time functions in postgresql
PostgreSQL supports the full set of SQL date and time types, shown in Table 8.9. The operations available on these data types are described in Section 9.9. Dates are counted according to the Gregorian calendar, even in years before that calendar was introduced (see Section B.6 for more information).
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 (no time zone)||4713 BC||294276 AD||1 microsecond|
|timestamp [ ( p ) ] with time zone||8 bytes||both date and time, with time zone||4713 BC||294276 AD||1 microsecond|
|date||4 bytes||date (no time of day)||4713 BC||5874897 AD||1 day|
|time [ ( p ) ] [ without time zone ]||8 bytes||time of day (no date)||00:00:00||24:00:00||1 microsecond|
|time [ ( p ) ] with time zone||12 bytes||time of day (no date), with time zone||00:00:00+1559||24:00:00-1559||1 microsecond|
|interval [ fields ] [ ( p ) ]||16 bytes||time interval||-178000000 years||178000000 years||1 microsecond|
The SQL standard requires that writing just timestamp be equivalent to timestamp without time zone , and PostgreSQL honors that behavior. timestamptz is accepted as an abbreviation for timestamp with time zone ; this is a PostgreSQL extension.
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.
The interval type has an additional option, which is to restrict the set of stored fields by writing one of these phrases:
Note that if both fields and p are specified, the fields must include SECOND , since the precision applies only to the seconds.
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.
8.5.1. Date/Time Input
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 day, month, 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 22.214.171.124 for more information. SQL requires the following syntax
where p is an optional precision specification giving the number of fractional digits in the seconds field. Precision can be specified for time , timestamp , and interval types, and can range from 0 to 6. If no precision is specified in a constant specification, it defaults to the precision of the literal value (but not more than 6 digits).
Table 8.10 shows some possible inputs for the date type.
Table 8.10. Date Input
|1999-01-08||ISO 8601; January 8 in any mode (recommended format)|
|January 8, 1999||unambiguous in any datestyle input mode|
|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 BC|
The time-of-day types are time [ ( p ) ] without time zone and time [ ( p ) ] with time zone . time alone 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 04:05:06.789-8||ISO 8601, with time zone as UTC offset|
|04:05:06-08:00||ISO 8601, with time zone as UTC offset|
|04:05-08:00||ISO 8601, with time zone as UTC offset|
|040506-08||ISO 8601, with time zone as UTC offset|
|040506+0730||ISO 8601, with fractional-hour time zone as UTC offset|
|040506+07:30:00||UTC offset specified to seconds (not allowed in ISO 8601)|
|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|
|-8:00:00||UTC offset for PST|
|-8:00||UTC offset for PST (ISO 8601 extended format)|
|-800||UTC offset for PST (ISO 8601 basic format)|
|-8||UTC offset for PST (ISO 8601 basic format)|
|zulu||Military abbreviation for UTC|
|z||Short form of zulu (also in ISO 8601)|
Refer to Section 8.5.3 for more information on how to specify time zones.
126.96.36.199. Time Stamps
Valid input for the time stamp types consists of the 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:
is a timestamp without time zone , while
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:
In a literal that has been determined 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 9.9.4).
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 time zone can be specified for the conversion using AT TIME ZONE .
188.8.131.52. Special Values
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 unchanged; 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 enclosed 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||date , timestamp||later than all other time stamps|
|-infinity||date , timestamp||earlier than all other time stamps|
|now||date , time , timestamp||current transaction’s start time|
|today||date , timestamp||midnight ( 00:00 ) today|
|tomorrow||date , timestamp||midnight ( 00:00 ) tomorrow|
|yesterday||date , timestamp||midnight ( 00:00 ) yesterday|
While the input strings now , today , tomorrow , and yesterday are fine to use in interactive SQL commands, they can have surprising behavior when the command is saved to be executed later, for example in prepared statements, views, and function definitions. The string can be converted to a specific time value that continues to be used long after it becomes stale. Use one of the SQL functions instead in such contexts. For example, CURRENT_DATE + 1 is safer than ‘tomorrow’::date .
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), traditional POSTGRES (Unix date format), or German. 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 generally only the date or time part in accordance with the given examples. However, the POSTGRES style outputs date-only values in ISO format.
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|
ISO 8601 specifies the use of uppercase letter T to separate the date and time. PostgreSQL accepts that format on input, but on output it uses a space rather than T , as shown above. This is for readability and for consistency with RFC 3339 as well as some other database systems.
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|
The date/time style 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 function to_char (see Section 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.
Although the date type cannot 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 can vary through the year with daylight-saving time boundaries.
To address these difficulties, we recommend using date/time types that contain both date and time when using time zones. We do not recommend 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 54.32). PostgreSQL uses the widely-used IANA time zone data for this purpose, so the same time zone names are also recognized by 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 can imply a set of daylight savings transition rules as well. The recognized abbreviations are listed in the pg_timezone_abbrevs view (see Section 54.31). You cannot set the configuration parameters TimeZone or log_timezone to 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, as described in Section B.5. This option is not normally preferable to using a named time zone, but it may be necessary if no suitable IANA time zone entry is available.
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). So 2014-06-04 12:00 EDT specifies that same time instant. But 2014-06-04 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 Moscow MSK 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 the EST example above, this is not necessarily the same as local civil time on that date.
In all cases, timezone names and abbreviations are recognized case-insensitively. (This is a change from PostgreSQL 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 (see Section B.4).
The TimeZone configuration parameter can be set in the file postgresql.conf , or in any of the other standard ways described in Chapter 20. There are also some special ways to set it:
The PGTZ environment variable is used by libpq clients to send a SET TIME ZONE command to the server upon connection.