8.5. Date/Time Types
Postgres Pro 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.5 for more information).
Table 8.9. Date/Time Types
|Name||Storage Size||Description||Low Value||High Value||Resolution|
|8 bytes||both date and time (no time zone)||4713 BC||294276 AD||1 microsecond / 14 digits|
|8 bytes||both date and time, with time zone||4713 BC||294276 AD||1 microsecond / 14 digits|
|4 bytes||date (no time of day)||4713 BC||5874897 AD||1 day|
|8 bytes||time of day (no date)||00:00:00||24:00:00||1 microsecond / 14 digits|
|12 bytes||times of day only, with time zone||00:00:00+1459||24:00:00-1459||1 microsecond / 14 digits|
|16 bytes||time interval||-178000000 years||178000000 years||1 microsecond / 14 digits|
The SQL standard requires that writing just
timestamp be equivalent to
timestamp without time zone, and Postgres Pro honors that behavior.
timestamptz is accepted as an abbreviation for
timestamp with time zone; this is a Postgres Pro extension.
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 values are stored as eight-byte integers (currently the default), microsecond precision is available over the full range of values. When
timestamp 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. When
timestamp 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 of
timestamp values to be represented than shown above: from 4713 BC up to 5874897 AD.
The same compile-time option also determines whether
interval values are stored as floating-point numbers or eight-byte integers. In the floating-point case, large
interval values degrade in precision as the size of the interval increases.
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.
interval type has an additional option, which is to restrict the set of stored fields by writing one of these phrases:
YEAR MONTH DAY HOUR MINUTE SECOND YEAR TO MONTH DAY TO HOUR DAY TO MINUTE DAY TO SECOND HOUR TO MINUTE HOUR TO SECOND MINUTE TO SECOND
Note that if both
p are specified, the
fields must include
SECOND, since the precision applies only to the seconds.
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
timestamp without time zone, and
timestamp with time zone should provide a complete range of date/time functionality required by any application.
reltime are lower precision types which are used internally. You are discouraged from using these types in applications; 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 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.
Postgres Pro 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
p) ] '
p is an optional precision specification giving the number of fractional digits in the seconds field. Precision can be specified for
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
Table 8.10. Date Input
|1999-01-08||ISO 8601; January 8 in any mode (recommended format)|
|January 8, 1999||unambiguous in any |
|1/8/1999||January 8 in |
|1/18/1999||January 18 in |
|01/02/03||January 2, 2003 in |
|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 |
|08-Jan-99||January 8, except error in |
|Jan-08-99||January 8, except error in |
|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 [ ( and
p) ] without time zone
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
|same as 04:05; AM does not affect value|
|same as 16:05; input hour must be <= 12|
|time zone specified by abbreviation|
|time zone specified by full name|
Table 8.12. Time Zone Input
|Abbreviation (for Pacific Standard Time)|
|Full time zone name|
|POSIX-style time zone specification|
|ISO-8601 offset for PST|
|ISO-8601 offset for PST|
|ISO-8601 offset for PST|
|Military abbreviation for UTC|
|Short form of |
Refer to Section 8.5.3 for more information on how to specify time zones.
184.108.40.206. 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
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 common 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 “-” symbol and time zone offset after the time. Hence, according to the standard,
TIMESTAMP '2004-10-19 10:23:54'
timestamp without time zone, while
TIMESTAMP '2004-10-19 10:23:54+02'
timestamp with time zone. Postgres Pro 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 determined to be
timestamp without time zone, Postgres Pro 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.
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
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).
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.
220.127.116.11. Special Values
Postgres Pro supports several special date/time input values for convenience, as shown in Table 8.13. The values
-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|
|1970-01-01 00:00:00+00 (Unix system time zero)|
|later than all other time stamps|
|earlier than all other time stamps|
|current transaction's start time|
The following SQL-compatible functions can also be used to obtain the current time value for the corresponding data type:
LOCALTIMESTAMP. The latter four accept an optional subsecond precision specification. (See Section 9.9.4.) Note that these are SQL functions and are not recognized in 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 (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
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 8601, SQL standard|
ISO 8601 specifies the use of uppercase letter
T to separate the date and time. Postgres Pro 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.
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 examples.
Table 8.15. Date Order Conventions
|Input Ordering||Example Output|
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. Postgres Pro 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.
Postgres Pro 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:
datetype cannot have an associated time zone, the
timetype 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 from UTC. It is therefore impossible 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 do not recommend using the type
time with time zone (though it is supported by Postgres Pro for legacy applications and for compliance with the SQL standard). Postgres Pro 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.
Postgres Pro 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_namesview (see Section 49.78). Postgres Pro 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-date rules as well. The recognized abbreviations are listed in the
pg_timezone_abbrevsview (see Section 49.77). 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 ZONEoperator.
In addition to the timezone names and abbreviations, Postgres Pro will accept POSIX-style time zone specifications of the form
STDis a zone abbreviation,
offsetis a numeric offset in hours west from UTC, and
DSTis an optional daylight-savings zone abbreviation, assumed to stand for one hour ahead of the given offset. For example, if
EST5EDTwere not already a recognized zone name, it would be accepted and would be functionally equivalent to United States East Coast time. In this syntax, a zone abbreviation can be a string of letters, or an arbitrary string surrounded by angle brackets (
<>). When a daylight-savings zone abbreviation is present, it is assumed to be used according to the same daylight-savings transition rules used in the IANA time zone database's
posixrulesentry. In a standard Postgres Pro installation,
posixrulesis 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
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. Postgres Pro 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.
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 locations west of Greenwich. Everywhere else, Postgres Pro follows the ISO-8601 convention that positive timezone offsets are east of Greenwich.
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/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 18. There are also some special ways to set it:
The SQL command
SET TIME ZONEsets the time zone for the session. This is an alternative spelling of
SET TIMEZONE TOwith a more SQL-spec-compatible syntax.
PGTZenvironment variable is used by libpq clients to send a
SET TIME ZONEcommand to the server upon connection.
8.5.4. Interval Input
interval values can be written using the following verbose syntax:
quantity is a number (possibly signed);
millennium, or abbreviations or plurals of these units;
direction can be
ago 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, if IntervalStyle is set to
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 the SQL standard, and are used for output when
IntervalStyle is set to
Interval values can also be written as ISO 8601 time intervals, using either the “format with designators” of the standard's section 18.104.22.168 or the “alternative format” of section 22.214.171.124. The format with designators looks like this:
unit...] [ T [
The string must start with a
P, and may include a
T that introduces the time-of-day units. The available unit abbreviations are given in Table 8.16. Units may be omitted, and may be specified in any order, but units smaller than a day must appear after
T. In particular, the meaning of
M depends on whether it is before or after
Table 8.16. ISO 8601 Interval Unit Abbreviations
|M||Months (in the date part)|
|M||Minutes (in the time part)|
In the alternative format:
days] [ T
the string must begin with
P, and a
T 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 a
fields specification, or when assigning a string to an interval column that was defined with a
fields specification, the interpretation of unmarked quantities depends on the
fields. For example
INTERVAL '1' YEAR is read as 1 year, whereas
INTERVAL '1' means 1 second. Also, field values “to the right” of the least significant field allowed by the
fields specification are silently discarded. For example, writing
INTERVAL '1 day 2:03:04' HOUR TO MINUTE results in dropping the seconds field, but not the day field.
According to the SQL 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. Postgres Pro 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. If
IntervalStyle is set to
sql_standard then a leading sign is considered to apply to all fields (but only if no additional signs appear). Otherwise the traditional Postgres Pro interpretation is used. To avoid ambiguity, it's recommended to attach an explicit sign to each field if any field is negative.
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 valid
Table 8.17. Interval Input
|1-2||SQL standard format: 1 year 2 months|
|3 4:05:06||SQL standard format: 3 days 4 hours 5 minutes 6 seconds|
|1 year 2 months 3 days 4 hours 5 minutes 6 seconds||Traditional Postgres format: 1 year 2 months 3 days 4 hours 5 minutes 6 seconds|
|P1Y2M3DT4H5M6S||ISO 8601 “format with designators”: same meaning as above|
|P0001-02-03T04:05:06||ISO 8601 “alternative format”: same meaning as above|
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. The months and days fields are integers while the seconds field can store fractions. Because intervals are usually created from constant strings or
timestamp subtraction, this storage method works well in most cases, but can cause unexpected results:
SELECT EXTRACT(hours from '80 minutes'::interval); date_part ----------- 1 SELECT EXTRACT(days from '80 hours'::interval); date_part ----------- 0
justify_hours are available for adjusting days and hours that overflow their normal ranges.
8.5.5. Interval Output
The output format of the interval type can be set to one of the four styles
iso_8601, using the command
SET intervalstyle. The default is the
postgres format. Table 8.18 shows examples of each output style.
sql_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 the
postgres style matches the output of PostgreSQL releases prior to 8.4 when the DateStyle parameter was set to
The output of the
postgres_verbose style matches the output of PostgreSQL releases prior to 8.4 when the
DateStyle parameter was set to non-
The output of the
iso_8601 style matches the “format with designators” described in section 126.96.36.199 of the ISO 8601 standard.
Table 8.18. Interval Output Style Examples
|Style Specification||Year-Month Interval||Day-Time Interval||Mixed Interval|
|1-2||3 4:05:06||-1-2 +3 -4:05:06|
|1 year 2 mons||3 days 04:05:06||-1 year -2 mons +3 days -04:05:06|
|@ 1 year 2 mons||@ 3 days 4 hours 5 mins 6 secs||@ 1 year 2 mons -3 days 4 hours 5 mins 6 secs ago|