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8.5. Date/Time Types |
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8.5. Date/Time Types PostgreSQL supports the full set of SQL date and time types, shown in Table 8.9, “Date/Time Types”. The operations available on these data types are described in Section 9.9, “Date/Time Functions and Operators”. Table 8.9. Date/Time Types
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. 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 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, Date/Time Support 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 4.1.2.5, “Constants of Other Types” 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. 8.5.1.1. Dates Table 8.10, “Date Input” shows some possible inputs for the date type. Table 8.10. Date Input
8.5.1.2. Times 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, “Time Input” and Table 8.12, “Time Zone Input”.) If a time zone is specified in the input for time without time zone, it is silently ignored. Table 8.11. Time Input
Table 8.12. Time Zone Input
Refer to Appendix B, Date/Time Support for a list of time zone names that are recognized for input. 8.5.1.3. Time Stamps 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 and 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 is supported. 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, “AT TIME ZONE”). 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. 8.5.1.4. Intervals interval values can be written with the following syntax: [@] quantity unit [quantity unit...] [direction] Where: quantity is a number (possibly signed); unit is 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_days and justify_hours are available for adjusting days and hours that overflow their normal periods. 8.5.1.5. Special Values PostgreSQL supports several special date/time input values for convenience, as shown in Table 8.13, “Special Date/Time Inputs”. 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
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, “Current Date/Time”.) Note however that these are SQL functions and are not recognized as data input strings. 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, 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, “Date/Time Output Styles” 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
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, “Date/Time Input” for how this setting also affects interpretation of input values.) Table 8.15, “Date Order Conventions” shows an example. Table 8.15. Date Order Conventions
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 function to_char (see Section 9.8, “Data Type Formatting Functions”) is also available as a more flexible way to format the 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 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:
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. The timezone configuration parameter can be set in the file postgresql.conf, or in any of the other standard ways described in Chapter 17, Server Configuration. There are also several special ways to set it:
Refer to Appendix B, Date/Time Support for a list of available time zones. 8.5.4. Internals PostgreSQL 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 Date conventions before the 19th century make for interesting reading, but are not consistent enough to warrant coding into a date/time handler. |