Date/Time Support
PostgreSQL uses an internal heuristic
parser for all date/time input support. Dates and times are input as
strings, and are broken up into distinct fields with a preliminary
determination of what kind of information may be in the
field. Each field is interpreted and either assigned a numeric
value, ignored, or rejected.
The parser contains internal lookup tables for all textual fields,
including months, days of the week, and time
zones.
This appendix includes information on the content of these
lookup tables and describes the steps used by the parser to decode
dates and times.
Date/Time Input Interpretation
The date/time type inputs are all decoded using the following procedure.
Break the input string into tokens and categorize each token as
a string, time, time zone, or number.
If the numeric token contains a colon (:>), this is
a time string. Include all subsequent digits and colons.
If the numeric token contains a dash (->), slash
(/>), or two or more dots (.>), this is
a date string which may have a text month.
If the token is numeric only, then it is either a single field
or an ISO 8601 concatenated date (e.g.,
19990113 for January 13, 1999) or time
(e.g., 141516 for 14:15:16).
If the token starts with a plus (+>) or minus
(->), then it is either a time zone or a special
field.
If the token is a text string, match up with possible strings.
Do a binary-search table lookup for the token
as either a special string (e.g., today),
day (e.g., Thursday),
month (e.g., January),
or noise word (e.g., at, on).
Set field values and bit mask for fields.
For example, set year, month, day for today,
and additionally hour, minute, second for now.
If not found, do a similar binary-search table lookup to match
the token with a time zone.
If still not found, throw an error.
When the token is a number or number field:
If there are eight or six digits,
and if no other date fields have been previously read, then interpret
as a concatenated date
(e.g.,
19990118 or 990118).
The interpretation is YYYYMMDD> or YYMMDD>.
If the token is three digits
and a year has already been read, then interpret as day of year.
If four or six digits and a year has already been read, then
interpret as a time (HHMM> or HHMMSS>).
If three or more digits and no date fields have yet been found,
interpret as a year (this forces yy-mm-dd ordering of the remaining
date fields).
Otherwise the date field ordering is assumed to follow the
DateStyle> setting: mm-dd-yy, dd-mm-yy, or yy-mm-dd.
Throw an error if a month or day field is found to be out of range.
If BC has been specified, negate the year and add one for
internal storage. (There is no year zero in the Gregorian
calendar, so numerically 1 BC becomes year
zero.)
If BC was not specified, and if the year field was two digits in length, then
adjust the year to four digits. If the field is less than 70, then add 2000,
otherwise add 1900.
Gregorian years AD 1-99 may be entered by using 4 digits with leading
zeros (e.g., 0099> is AD 99). Previous versions of
PostgreSQL accepted years with three
digits and with single digits, but as of version 7.0 the rules have
been tightened up to reduce the possibility of ambiguity.
Date/Time Key Words
shows the tokens that are
permissible as abbreviations for the names of the month.
Month Abbreviations
Month
Abbreviations
April
Apr
August
Aug
December
Dec
February
Feb
January
Jan
July
Jul
June
Jun
March
Mar
November
Nov
October
Oct
September
Sep, Sept
The month May has no explicit abbreviation, for obvious reasons.
shows the tokens that are
permissible as abbreviations for the names of the days of the
week.
Day of the Week Abbreviations
Day
Abbreviation
Sunday
Sun
Monday
Mon
Tuesday
Tue, Tues
Wednesday
Wed, Weds
Thursday
Thu, Thur, Thurs
Friday
Fri
Saturday
Sat
shows the tokens that serve
various modifier purposes.
Date/Time Field Modifiers
Identifier
Description
ABSTIME
Key word ignored
AM
Time is before 12:00
AT
Key word ignored
JULIAN>, JD>, J>
Next field is Julian Day
ON
Key word ignored
PM
Time is on or after 12:00
T
Next field is time
The key word ABSTIME is ignored for historical
reasons: In very old releases of
PostgreSQL, invalid values of type abstime
were emitted as Invalid Abstime. This is no
longer the case however and this key word will likely be dropped in
a future release.
time zone
abbreviations
shows the time zone
abbreviations recognized by PostgreSQL
in date/time input values.
PostgreSQL uses internal tables
for time zone input decoding, since there is no standard
operating system interface to provide access to general,
cross-time zone information. The underlying operating system
is used to provide time zone information for
output, however.
Keep in mind also that the time zone names
recognized by SET TIMEZONE> are operating-system
dependent and may have little to do with . For example, some systems
recognize values like 'Europe/Rome'> in SET
TIMEZONE>.
The table is organized by time zone offset from UTC>,
rather than alphabetically. This is intended to facilitate
matching local usage with recognized abbreviations for cases where
these might differ.
Time Zone Abbreviations
Time Zone
Offset from UTC
Description
NZDT
+13:00
New Zealand Daylight-Saving Time
IDLE
+12:00
International Date Line, East
NZST
+12:00
New Zealand Standard Time
NZT
+12:00
New Zealand Time
AESST
+11:00
Australia Eastern Summer Standard Time
ACSST
+10:30
Central Australia Summer Standard Time
CADT
+10:30
Central Australia Daylight-Saving Time
SADT
+10:30
South Australian Daylight-Saving Time
AEST
+10:00
Australia Eastern Standard Time
EAST
+10:00
East Australian Standard Time
GST
+10:00
Guam Standard Time, Russia zone 9
LIGT
+10:00
Melbourne, Australia
SAST
+09:30
South Australia Standard Time
CAST
+09:30
Central Australia Standard Time
AWSST
+09:00
Australia Western Summer Standard Time
JST
+09:00
Japan Standard Time, Russia zone 8
KST
+09:00
Korea Standard Time
MHT
+09:00
Kwajalein Time
WDT
+09:00
West Australian Daylight-Saving Time
MT
+08:30
Moluccas Time
AWST
+08:00
Australia Western Standard Time
CCT
+08:00
China Coastal Time
WADT
+08:00
West Australian Daylight-Saving Time
WST
+08:00
West Australian Standard Time
JT
+07:30
Java Time
ALMST
+07:00
Almaty Summer Time
WAST
+07:00
West Australian Standard Time
CXT
+07:00
Christmas (Island) Time
MMT
+06:30
Myanmar Time
ALMT
+06:00
Almaty Time
MAWT
+06:00
Mawson (Antarctica) Time
IOT
+05:00
Indian Chagos Time
MVT
+05:00
Maldives Island Time
TFT
+05:00
Kerguelen Time
AFT
+04:30
Afghanistan Time
EAST
+04:00
Antananarivo Summer Time
MUT
+04:00
Mauritius Island Time
RET
+04:00
Reunion Island Time
SCT
+04:00
Mahe Island Time
IRT, IT
+03:30
Iran Time
EAT
+03:00
Antananarivo, Comoro Time
BT
+03:00
Baghdad Time
EETDST
+03:00
Eastern Europe Daylight-Saving Time
HMT
+03:00
Hellas Mediterranean Time (?)
BDST
+02:00
British Double Standard Time
CEST
+02:00
Central European Summer Time
CETDST
+02:00
Central European Daylight-Saving Time
EET
+02:00
Eastern European Time, Russia zone 1
FWT
+02:00
French Winter Time
IST
+02:00
Israel Standard Time
MEST
+02:00
Middle European Summer Time
METDST
+02:00
Middle Europe Daylight-Saving Time
SST
+02:00
Swedish Summer Time
BST
+01:00
British Summer Time
CET
+01:00
Central European Time
DNT
+01:00
Dansk Normal Tid
FST
+01:00
French Summer Time
MET
+01:00
Middle European Time
MEWT
+01:00
Middle European Winter Time
MEZ
+01:00
Mitteleuropäische Zeit>
NOR
+01:00
Norway Standard Time
SET
+01:00
Seychelles Time
SWT
+01:00
Swedish Winter Time
WETDST
+01:00
Western European Daylight-Saving Time
GMT
00:00
Greenwich Mean Time
UT
00:00
Universal Time
UTC
00:00
Universal Coordinated Time
Z
00:00
Same as UTC
ZULU
00:00
Same as UTC
WET
00:00
Western European Time
WAT
-01:00
West Africa Time
FNST
-01:00
Fernando de Noronha Summer Time
FNT
-02:00
Fernando de Noronha Time
BRST
-02:00
Brasilia Summer Time
NDT
-02:30
Newfoundland Daylight-Saving Time
ADT
-03:00
Atlantic Daylight-Saving Time
AWT
-03:00
(unknown)
BRT
-03:00
Brasilia Time
NFT
-03:30
Newfoundland Standard Time
NST
-03:30
Newfoundland Standard Time
AST
-04:00
Atlantic Standard Time (Canada)
ACST
-04:00
Atlantic/Porto Acre Summer Time
EDT
-04:00
Eastern Daylight-Saving Time
ACT
-05:00
Atlantic/Porto Acre Standard Time
CDT
-05:00
Central Daylight-Saving Time
EST
-05:00
Eastern Standard Time
CST
-06:00
Central Standard Time
MDT
-06:00
Mountain Daylight-Saving Time
MST
-07:00
Mountain Standard Time
PDT
-07:00
Pacific Daylight-Saving Time
AKDT
-08:00
Alaska Daylight-Saving Time
PST
-08:00
Pacific Standard Time
YDT
-08:00
Yukon Daylight-Saving Time
AKST
-09:00
Alaska Standard Time
HDT
-09:00
Hawaii/Alaska Daylight-Saving Time
YST
-09:00
Yukon Standard Time
MART
-09:30
Marquesas Time
AHST
-10:00
Alaska/Hawaii Standard Time
HST
-10:00
Hawaii Standard Time
CAT
-10:00
Central Alaska Time
NT
-11:00
Nome Time
IDLW
-12:00
International Date Line, West
Australian Time Zones
There are three naming conflicts between Australian time zone
names and time zone names commonly used in North and South America:
ACST, CST, and
EST. If the run-time option
australian_timezones is set to true then
ACST, CST,
EST, and SAT are interpreted
as Australian time zone names, as shown in . If it is false (which is the
default), then ACST, CST,
and EST are taken as American time zone names,
and SAT is interpreted as a noise word
indicating Saturday.
Australian Time Zone Abbreviations
Time Zone
Offset from UTC
Description
ACST
+09:30
Central Australia Standard Time
CST
+10:30
Australian Central Standard Time
EST
+10:00
Australian Eastern Standard Time
SAT
+09:30
South Australian Standard Time
History of Units
The Julian Date was invented by the French scholar
Joseph Justus Scaliger (1540-1609)
and probably takes its name from Scaliger's father,
the Italian scholar Julius Caesar Scaliger (1484-1558).
Astronomers have used the Julian period to assign a unique number to
every day since 1 January 4713 BC. This is the so-called Julian Date
(JD). JD 0 designates the 24 hours from noon UTC on 1 January 4713 BC
to noon UTC on 2 January 4713 BC.
The Julian Date
is different from the Julian
Calendar
. The Julian calendar
was introduced by Julius Caesar in 45 BC. It was in common use
until the year 1582, when countries started changing to the Gregorian
calendar. In the Julian calendar, the tropical year is
approximated as 365 1/4 days = 365.25 days. This gives an error of
about 1 day in 128 years.
The accumulating calendar error prompted
Pope Gregory XIII to reform the calendar in accordance with
instructions from the Council of Trent.
In the Gregorian calendar, the tropical year is approximated as
365 + 97 / 400 days = 365.2425 days. Thus it takes approximately 3300
years for the tropical year to shift one day with respect to the
Gregorian calendar.
The approximation 365+97/400 is achieved by having 97 leap years
every 400 years, using the following rules:
Every year divisible by 4 is a leap year.
However, every year divisible by 100 is not a leap year.
However, every year divisible by 400 is a leap year after all.
So, 1700, 1800, 1900, 2100, and 2200 are not leap years. But 1600,
2000, and 2400 are leap years.
By contrast, in the older Julian calendar all years divisible by 4 are leap
years.
The papal bull of February 1582 decreed that 10 days should be dropped
from October 1582 so that 15 October should follow immediately after
4 October.
This was observed in Italy, Poland, Portugal, and Spain. Other Catholic
countries followed shortly after, but Protestant countries were
reluctant to change, and the Greek orthodox countries didn't change
until the start of the 20th century.
The reform was observed by Great Britain and Dominions (including what is
now the USA) in 1752.
Thus 2 September 1752 was followed by 14 September 1752.
This is why Unix systems have the cal program
produce the following:
$ cal 9 1752
September 1752
S M Tu W Th F S
1 2 14 15 16
17 18 19 20 21 22 23
24 25 26 27 28 29 30
The SQL standard states that Within the definition of a
datetime literal
, the datetime
value
s are constrained by the natural rules for dates and
times according to the Gregorian calendar
. Dates between
1752-09-03 and 1752-09-13, although eliminated in some countries
by Papal fiat, conform to natural rules
and are
hence valid dates.
Different calendars have been developed in various parts of the
world, many predating the Gregorian system.
For example,
the beginnings of the Chinese calendar can be traced back to the 14th
century BC. Legend has it that the Emperor Huangdi invented the
calendar in 2637 BC.
The People's Republic of China uses the Gregorian calendar
for civil purposes. The Chinese calendar is used for determining
festivals.