What is a good, simple way to compute ISO 8601 week number?

Question

Suppose I have a date, i.e. year, month and day, as integers. What's a good (correct), concise and fairly readable algorithm for computing the ISO 8601 week number of the week the given date falls into? I have come across some truly horrendous code that makes me think surely there must be a better way.

I'm looking to do this in Java but psuedocode for any kind of object-oriented language is fine.

Answer 1

tl;dr

LocalDate.of( 2015 , 12 , 30 )
         .get ( 
             IsoFields.WEEK_OF_WEEK_BASED_YEAR 
         )

53

…or…

org.threeten.extra.YearWeek.from (
    LocalDate.of( 2015 , 12 , 30 )
)

2015-W53

java.time

Support for the ISO 8601 week is now built into Java 8 and later, in the java.time framework. Avoid the old and notoriously troublesome java.util.Date/.Calendar classes as they have been supplanted by java.time.

These new java.time classes include LocalDate for date-only value without time-of-day or time zone. Note that you must specify a time zone to determine ‘today’ as the date is not simultaneously the same around the world.

ZoneId zoneId = ZoneId.of ( "America/Montreal" );
ZonedDateTime now = ZonedDateTime.now ( zoneId );

Or specify the year, month, and day-of-month as suggested in the Question.

LocalDate localDate = LocalDate.of( year , month , dayOfMonth );

The IsoFields class provides info according to the ISO 8601 standard including the week-of-year for a week-based year.

int calendarYear = now.getYear();
int weekNumber = now.get ( IsoFields.WEEK_OF_WEEK_BASED_YEAR );
int weekYear = now.get ( IsoFields.WEEK_BASED_YEAR ); 

Near the beginning/ending of a year, the week-based-year may be ±1 different than the calendar-year. For example, notice the difference between the Gregorian and ISO 8601 calendars for the end of 2015: Weeks 52 & 1 become 52 & 53.

ThreeTen-Extra — YearWeek

The YearWeek class represents both the ISO 8601 week-based year number and the week number together as a single object. This class is found in the ThreeTen-Extra project. The project adds functionality to the java.time classes built into Java.

ZoneId zoneId = ZoneId.of ( "America/Montreal" );
YearWeek yw = YearWeek.now( zoneId ) ;

Generate a YearWeek from a date.

YearWeek yw = YearWeek.from (
    LocalDate.of( 2015 , 12 , 30 )
)

This class can generate and parse strings in standard ISO 8601 format.

String output = yw.toString() ;

2015-W53

YearWeek yw = YearWeek.parse( "2015-W53" ) ;

You can extract the week number or the week-based-year number.

int weekNumber = yw.getWeek() ;
int weekBasedYearNumber = yw.getYear() ;

You can generate a particular date (LocalDate) by specifying a desired day-of-week to be found within that week. To specify the day-of-week, use the DayOfWeek enum built into Java 8 and later.

LocalDate ld = yw.atDay( DayOfWeek.WEDNESDAY ) ;

---

About java.time

The java.time framework is built into Java 8 and later. These classes supplant the troublesome old legacy date-time classes such as java.util.Date, Calendar, & SimpleDateFormat.

To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations. Specification is JSR 310.

The Joda-Time project, now in maintenance mode, advises migration to the java.time classes.

You may exchange java.time objects directly with your database. Use a JDBC driver compliant with JDBC 4.2 or later. No need for strings, no need for java.sql.* classes.

Where to obtain the java.time classes?

• Java SE 8, Java SE 9, Java SE 10, Java SE 11, and later - Part of the standard Java API with a bundled implementation.
  • Java 9 adds some minor features and fixes.
• Java SE 6 and Java SE 7
  • Most of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport.
• Android
  • Later versions of Android bundle implementations of the java.time classes.
  • For earlier Android (<26), the ThreeTenABP project adapts ThreeTen-Backport (mentioned above). See How to use ThreeTenABP….

The ThreeTen-Extra project extends java.time with additional classes. This project is a proving ground for possible future additions to java.time. You may find some useful classes here such as Interval, YearWeek, YearQuarter, and more.

Answer 2

I believe you can use the Calendar object (just set FirstDayOfWeek to Monday and MinimalDaysInFirstWeek to 4 to get it to comply with ISO 8601) and call get(Calendar.WEEK_OF_YEAR).