object

# iso8601¶

ISO 8601 (and European civil calendar) compliant library of date predicates.

Author: Daniel L. Dudley
Version: 1.02
Date: 2014/11/3
Compilation flags:
static, context_switching_calls

(no dependencies on other entities)

## Public interface¶

### date/4¶

Get the system date and/or its Julian Day # or convert a Julian Day # to/from given date parts.

Compilation flags:
static
Template:
date(JD,Year,Month,Day)
JD - Julian day serial number
Year - 0 or negative if converted BC year, positive otherwise
Month - Normally an integer between 1 and 12 inclusive
Day - Normally an integer between 1 and 31 inclusive depending upon month
Mode and number of proofs:
date(?integer,?integer,?integer,?integer) - zero_or_one
Examples:
Current date (i.e., today)
date(JD,Year,Month,Day)
JD=2453471,Year=2005,Month=4,Day=10
Convert a date to its Julian day number
date(JD,2000,2,29)
JD=2451604
Convert a Julian day number to its date
date(2451604,Year,Month,Day)
Year=2000,Month=2,Day=29
What is the date of day # 60 in year 2000?
date(JD,2000,1,60)
JD=2451604
What is the Julian of the 1st day prior to 2000-1-1?
date(JD,2000,1,0)
JD=2451544
What is the Julian of the 60th day prior to 2000-1-1?
date(JD,2000,1,-59)
JD=2451485
date(JD,1900,2,29)
JD=2415080
This is the correct date!
date(2415080,Year,Month,Day)
Year=1900,Month=3,Day=1

### date/5¶

Ditto date/4 + get/check its day-of-week #.

Compilation flags:
static
Template:
date(JD,Year,Month,Day,DoW)
JD - Julian day serial number
Year - 0 or negative if converted BC year, positive otherwise
Month - Normally an integer between 1 and 12 inclusive
Day - Normally an integer between 1 and 31 inclusive depending upon month
DoW - Day of week, where Monday=1, Tuesday=2, …, Sunday=7
Mode and number of proofs:
date(?integer,?integer,?integer,?integer,?integer) - zero_or_one
Examples:
Get the Julian and the day-of-week # of a date
date(JD,2000,2,29,DoW)
JD=2451604,DoW=2
Check the validity of a given date (day-of-week is 2, not 4)
date(_,2002,3,5,4)
no
Get the Julian day of a given date if it is a Sunday
date(JD,2004,2,29,7)
JD=2453065
Get the date and day-of-week # of a Julian
date(2451545,Year,Month,Day,DoW)
Year=2000,Month=1,Day=1,DoW=6

### date/6¶

Ditto date/5 + get/check its week #.

Compilation flags:
static
Template:
date(JD,Year,Month,Day,DoW,Week)
JD - Julian day serial number
Year - 0 or negative if converted BC year, positive otherwise
Month - Normally an integer between 1 and 12 inclusive
Day - Normally an integer between 1 and 31 inclusive depending upon month
DoW - Day of week, where Monday=1, Tuesday=2, …, Sunday=7
Week - Compound term, week(WeekNo,ActualYear), of a day
Mode and number of proofs:
date(?integer,?integer,?integer,?integer,?integer,?compound) - zero_or_one
Examples:
Get the day-of-week and week number of a date
date(_,2000,1,1,DoW,Week)
DoW=6,Week=week(52,1999)
Get the week number and year of this week
date(_,_,_,_,_,Week)
Week=week(7,2004)
Get the Julian number and the week of a date if it is a Sunday
date(JD,2004,2,29,7,Week)
JD=2453065,Week=week(9,2004)
Get the day-of-week and week of a Julian day number
date(2453066,_,_,_,DoW,Week)
DoW=1,Week=week(10,2004)
Check that given date data matches
date(_,2004,3,1,1,week(10,2004))
yes
What is the date of a day of week (default is 1) in given week # and year?
date(_,Year,Month,Day,DoW,week(26,2004))
Year=2004,Month=6,Day=21,DoW=1
Ditto for Sunday
date(_,Year,Month,Day,7,week(1,2005))
Year=2005,Month=1,Day=9
Ditto for Tuesday in following week
date(_,Year,Month,Day,9,week(1,2005))
Year=2005,Month=1,Day=11
Ditto for Thursday in the prior week
date(_,Year,Month,Day,4,week(0,2005))
Year=2004,Month=12,Day=30
Ditto for Tuesday two weeks prior
date(_,Year,Month,Day,2,week(-1,2005))
Year=2004,Month=12,Day=21
Ditto for Saturday
date(_,Year,Month,Day,6,week(53,2004))
Year=2005,Month=1,Day=1
Ditto for Monday (note automatic compensation of nonexistent week number)
date(_,Year,Month,Day,1,week(60,2004))
Year=2005,Month=2,Day=14

### date/7¶

Ditto date/6 + get/check its day-of-year #.

Compilation flags:
static
Template:
date(JD,Year,Month,Day,DoW,Week,DoY)
JD - Julian day serial number
Year - 0 or negative if converted BC year, positive otherwise
Month - Normally an integer between 1 and 12 inclusive
Day - Normally an integer between 1 and 31 inclusive depending upon month
DoW - Day of week, where Monday=1, Tuesday=2, …, Sunday=7
Week - Compound term, week(WeekNo,ActualYear), of a day
DoY - Day of year (NB! calendar year, not week # year)
Mode and number of proofs:
date(?integer,?integer,?integer,?integer,?integer,?compound,?integer) - zero_or_one
Examples:
Get the date and day-of-year of a Julian number
date(2451649,Year,Month,Day,_,_,DoY)
Year=2000,Month=4,Day=14,DoY=105
Get the Julian number, week number and day-of-year of a date, confirming that it is a Sunday
date(JD,2004,2,29,7,Week,DoY)
JD=2453065,Week=week(9,2004),DoY=60
Confirm that a date is, in fact, a specific day-of-year
date(_,2004,3,1,_,_,61)
yes
Get the Julian number, week day and day-of-year of a date
date(JD,2004,10,18,DoW,_,DoY)
JD=2453297,DoW=1,DoY=292
Get today’s day-of-year
date(_,_,_,_,_,_,DoY)
DoY=54
Get all missing date data (excl. Julian number) for the 60th calendar day of 2004
date(_,2004,Month,Day,DoW,Week,60)
Month=2,Day=29,DoW=7,Week=week(9,2004)
Match given date data and, if true, return the missing data (excl. Julian number)
date(_,2004,3,Day,DoW,Week,61)
Day=1,DoW=1,Week=week(10,2004)
Ditto (the 61st day-of-year cannot be both day 1 and 2 of the month)
date(_,2004,_,2,_,_,61)
no

### date_string/3¶

Conversion between an ISO 8601 compliant date string and its components (truncated and expanded date representations are currently unsupported). Note that date components are not validated; that is the caller’s responsibility!

Compilation flags:
static
Template:
date_string(Format,Components,String)
Format - ISO 8601 format
Components - When bound and String is free, either a Julian number or a [Year,Month,Day] term; it binds to the system day/date if free When free and String is bound, it binds to an integer list representing the numeric elements of String
String - ISO 8601 formatted string correspondent to Components
Mode and number of proofs:
date_string(+atom,+integer,?atom) - zero_or_one
date_string(+atom,?list,?atom) - zero_or_one
Examples:
Date, complete, basic (section 5.2.1.1)
date_string('YYYYMMDD',[2004,2,29],String)
String='20040229'
Date, complete, basic (section 5.2.1.1)
date_string('YYYYMMDD',Components,'20040229')
Components=[2004,2,29]
Date, complete, extended (section 5.2.1.1)
date_string('YYYY-MM-DD',[2003,12,16],String)
String='2003-12-16'
Date, complete, extended (section 5.2.1.1)
date_string('YYYY-MM-DD',Components,'2003-12-16')
Components=[2003,12,16]
Date, complete, extended (section 5.2.1.1)
date_string('YYYY-MM-DD',_,String)
String='2004-02-17'
Date, complete, extended (section 5.2.1.1)
date_string('YYYY-MM-DD',Components,'2004-02-17')
Components=[2004,2,17]
Date, reduced, month (section 5.2.1.2 a)
date_string('YYYY-MM',[2004,9,18],String)
String='2004-09'
Date, reduced, month (section 5.2.1.2 a)
date_string('YYYY-MM',Components,'2004-09')
Components=[2004,9]
Date, reduced, year (section 5.2.1.2 b)
date_string('YYYY',[1900,7,24],String)
String='1900'
Date, reduced, year (section 5.2.1.2 b)
date_string('YYYY',Components,'1900')
Components=[1900]
Date, reduced, century (section 5.2.1.2 c)
date_string('YY',2456557,String)
String='20'
Date, reduced, century (section 5.2.1.2 c)
date_string('YY',Components,'20')
Components=[20]
Date, ordinal, complete (section 5.2.2.1)
date_string('YYYYDDD',[2005,3,25],String)
String='2005084'
Date, ordinal, complete (section 5.2.2.1)
date_string('YYYYDDD',Components,'2005084')
Components=[2005,84]
Date, ordinal, extended (section 5.2.2.1)
date_string('YYYY-DDD',[1854,12,4],String)
String='1854-338'
Date, ordinal, extended (section 5.2.2.1)
date_string('YYYY-DDD',Components,'1854-338')
Components=[1854,338]
Week, complete, basic (section 5.2.3.1)
date_string('YYYYWwwD',[2000,1,2],String)
String='1999W527'
Week, complete, basic (section 5.2.3.1)
date_string('YYYYWwwD',Components,'1999W527')
Components=[1999,52,7]
Week, complete, extended (section 5.2.3.1)
date_string('YYYY-Www-D',[2003,12,29],String)
String='2004-W01-1'
Week, complete, extended (section 5.2.3.1)
date_string('YYYY-Www-D',Components,'2004-W01-1')
Components=[2004,1,1]
Week, complete, extended (section 5.2.3.1)
date_string('YYYY-Www-D',2453167,String)
String='2004-W24-4'
Week, complete, extended (section 5.2.3.1)
date_string('YYYY-Www-D',Components,'2004-W24-4')
Components=[2004,24,4]
Week, reduced, basic (section 5.2.3.2)
date_string('YYYYWww',[2004,2,29],String)
String='2004W09'
Week, reduced, basic (section 5.2.3.2)
date_string('YYYYWww',Components,'2004W09')
Components=[2004,9]
Week, reduced, extended (section 5.2.3.2)
date_string('YYYY-Www',[2004,2,29],String)
String='2004-W09'
Week, reduced, extended (section 5.2.3.2)
date_string('YYYY-Www',Components,'2004-W09')
Components=[2004,9]

### valid_date/3¶

Validate a given date in the Gregorian calendar.

Compilation flags:
static
Template:
valid_date(Year,Month,Day)
Mode and number of proofs:
valid_date(+integer,+integer,+integer) - zero_or_one
Examples:
Yes, the recent millenium was a leap year
valid_date(2000,2,29)
yes
2004 was also a leap year
valid_date(2004,2,29)
yes
Only 30 days in April
valid_date(2004,4,31)
no
1 BC was a leap year
valid_date(-1,2,29)
yes

### leap_year/1¶

Succeed if given year is a leap year in the Gregorian calendar.

Compilation flags:
static
Template:
leap_year(Year)
Year - The Gregorian calendar year to investigate. If free, it binds to the system year
Mode and number of proofs:
leap_year(?integer) - zero_or_one
Examples:
No, the prior centenary was not a leap year
leap_year(1900)
no
The recent millenium
leap_year(2000)
yes
This year
leap_year(Year)
Year=2004
This year (equivalent to prior query)
leap_year(_)
yes
Next centennial
leap_year(2100)
no
Year 0, equivalent to 1 BC
leap_year(0)
yes
1 BC
leap_year(-1)
yes
4 BC
leap_year(-4)
no
5 BC
leap_year(-5)
yes

### calendar_month/3¶

Compute a calendar month.

Compilation flags:
static
Template:
calendar_month(Year,Month,Calendar)
Year - The calendar year
Month - The calendar month
Calendar - A compound term, m/3, composed of three main arguments specifying year, month, and a list of week and week day numbers (calendar body).
Mode and number of proofs:
calendar_month(?integer,?integer,-compound) - zero_or_one
Examples:
Compute the calendar of March, 2005
calendar_month(2005,3,Calendar)
Calendar=m(2005,3,[w(9,[0,1,2,3,4,5,6]),w(10,[7,8,9,10,11,12,13]),w(11,[14,15,16,17,18,19,20]),w(12,[21,22,23,24,25,26,27]),w(13,[28,29,30,31,0,0,0]),w(0,[0,0,0,0,0,0,0])])

### easter_day/3¶

Compute a Gregorian Easter Sunday.

Compilation flags:
static
Template:
easter_day(Year,Month,Day)
Year - Integer specifying the year to be investigated
Month - Month in which Easter Sunday falls for given year
Day - Day of month in which Easter Sunday falls for given year
Mode and number of proofs:
easter_day(?integer,-integer,-integer) - zero_or_one
Examples:
Compute Easter Sunday for a particular year
easter_day(2006,Month,Day)
Month=4,Day=16
Compute Easter Sunday for the current year
easter_day(Year,Month,Day)
Year=2005,Month=3,Day=27

(none)

(none)

(none)

## Remarks¶

• Scope: This object currently provides a powerful, versatile and efficient set of date-handling predicates, which–thanks to Logtalk–may be used as is on a wide range of Prolog compilers. Besides taking time to familiarize oneself with each predicate, the user should take note of the following information.
• Validation of dates: Date parts are not validated–that is the caller’s responsibility! However, not being quite heartless yet, we do provide a predicate for this purpose.
• Date arithmetic: Many of the examples illustrate a simplified method of doing date arithmetic. Note, however, that we do not generally recommend this practice–it is all too easy to make mistakes. The safest way of finding the day difference between two dates is to first convert the dates to their Julian day numbers and then subtract one from the other. Similarly, the safe way to add or subtract a day offset to a particular date is to first convert the date to its Julian day number, add or subtract the day offset, and then convert the result to its corresponding date.
• BC years: ISO 8601 specifies that the Gregorian calendar be used, yet requires that years prior to 1 AD be handled arithmetically, i.e., the year we know as 1 BC is year 0, 2 BC is year -1, 3 BC is year -2 and so on. We do not follow ISO 8601 with regard to the handling of BC years. Our date predicates will accept and interpret an input year 0 as 1 BC; however, a negative year, Year, should always be interpreted as abs(Year) =:= Year BC. We believe that the average person will find our handling of BC years more user-friendly than the ISO 8601 one, but we encourage feedback from users with a view to a possible change in future versions.
• Week numbers: It is possible for a day (date) to have a week number that belongs to another year. Up to three of the first days of a calendar year may belong to the last week (number) of the prior calendar year, and up to three days of the last days of a calendar year may belong to the first week (number) of the next calendar year. It for this reason that the Week parameter in date/6-7 is a compound term, namely week(WeekNo,ActualYear).
• Computation of Gregorian Easter Sunday: The algorithm is based upon the “Gaussian rule”. Proleptic use is limited to years > 1582 AD, that is, after the introduction of the Gregorian calendar.
• Some Christian feast day offsets from Easter Sunday: Carnival Monday: -48 days, Mardi Gras (Shrove Tuesday): -47 days, Ash Wednesday: -46 days, Palm Sunday: -7 days, Easter Friday: -2 days, Easter Saturday: -1 day, Easter Monday: +1 day, Ascension of Christ: +39 days, Whitsunday: +49 days, Whitmonday: +50 days, Feast of Corpus Christi: +60 days.