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Date::WeekOfYear - Simple routine to return the ISO 8601 week of the year (as well as the ISO week year)


  use Date::WeekOfYear;

  # Get the week number (and year for the end/start of year transitions)
  my ($wkNo, $year) = WeekOfYear();

  # Get the week number for the time passed in time_stamp
  my ($wkNo, $year) = WeekOfYear($time_stamp);

  # Use the data for someThing ...
  my $logFile = "/someDir/$year/someApp_$wkNo.log"

  # Only want the week number, don't care which year in the week around
  # the end/start of the year !
  my $weekNo = WeekOfYear();

  # Handle years outside of perls localtime functions - 04/01/2090
  my ($wkNo, $year) = WeekOfYear({ year => 2090, month => 1, day => 4});

  # or in ISO-8601 format YYYY-Wnn
  my $iso_8601_wkno = WeekOfYear({ year => 2090, month => 1, day => 4}, WOY_ISO_MODE);

  # Week number as per pre V1.5
  my ($wkNo, $year) = WeekOfYear($time_stamp, WOY_OLD_MODE);


Date::WeekOfYear is small and efficient. The only purpose is to return the week of the year. This can be called in either a scalar or list context.

In a scalar context, just the week number is returned (the year starts at week 1).

In a list context, both the week number and the year (YYYY) are returned. This ensures that you know which year the week number relates too. This is only an issue in the week where the year changes (ie depending on the day you can be in either week 52, week 53 or week 1.

NOTE The year returned is not always the same as the Gregorian year for that day for further details see ISO 8601.

To obtain the old functionality, a mode is also passed, WOY_OLD_MODE. Note you need to use the ':mode' or ':all' tags to use to gain access to WOY_OLD_MODE.

If mode WOY_ISO_MODE is used the output will be in the ISO 8601 format YYYY-Wxx where YYYY is the year and xx is the two digit week number and 'W' denotes week.


If a second argument, the mode, is provided then either the pre version 1.5 mode is activated or the output is format as per ISO 8601 as YYYY-Wxx

The modes are WOY_OLD_MODE and WOY_ISO_MODE


Used to select the old mode, eg

 ($wkNo, $year) = WeekOfYear($time_stamp, WOY_OLD_MODE);


Use to select the output formatted as YYYY-Wxx, eg

 $iso_8601_wkno = WeekOfYear({ year => 2090, month => 1, day => 4}, WOY_ISO_MODE);



The function, see SYNOPSIS above


These other functions are available if the use tag ':all' is used, eg:

 use Date::WeekOfYear ':all';


Returns the day of the year, 1 being the first day. The last day is either 365 or 366, the latter if the year is a leap year. Expected arguments are the year, month and day as numeric values. The year is expected as yyyy, the month as 1 to 12 for Jan to Dec and the day of the month.


Returns true (1) if the year is a leap year, otherwise false (0). The expected argument is the year as a numeric value of format yyyy, eg 2014.

In general terms the algorithm for calculating a leap year is as follows...

A year will be a leap year if it is divisible by 4 but not by 100. If a year is divisible by 4 and by 100, it is not a leap year unless it is also divisible by 400. Thus years such as 1996, 1992, 1988 and so on are leap years because they are divisible by 4 but not by 100. For century years, the 400 rule is important. Thus, century years 1900, 1800 and 1700 while all still divisible by 4 are also exactly divisible by 100. As they are not further divisible by 400, they are not leap years.


Returns the week_day of the 1st of January for the year in question. The week_day is a numeric value indicating the day and differs from that returned by the core function localtime() in that Sunday is 7 rather than 0.

The returned values are:

 1=Mon, 2=Tue, 3=Wed, 4=Thu, 5=Fri, 6=Sat and 7=Sun

The expected argument is the year in yyyy format, eg 2014.


week_day takes the year (yyyy eg 2014), month (1 to 12) and month_day as arguments and returns the week day.

The week day returned is an integer representing the day of the week where:

 1 = Monday
 2 = Tuesday
 3 = Wednesday
 4 = Thursday
 5 = Friday
 6 = Saturday
 7 = Sunday.

Note this is similar to that returned by localtime except that Sunday is 7 rather than 0


week_number takes the year (yyyy eg 2014), month (1 to 12) and month_day as arguments and returns the week number as defined by ISO-8061. That is week 1 starts on a Monday and contains the first Thursday in the year. As a result week 1 can start in the previous year and a year can have either 52 or 53 weeks.


As of version 1.5 the ISO 8601 week number is calculated. For backwards compatibility a flag can be passed after the time to give the previous functionality.

For example:

  my $weekNo = WeekOfYear(undef, 1);  # Week number now in pre ISO 8601 mode
  my $weekNo = WeekOfYear($the_time, 1);  # Week number for $the_time in pre ISO 8601 mode

ISO 8601

Weeks in a Gregorian calendar year can be numbered for each year. This style of numbering is commonly used (for example, by schools and businesses) in some European and Asian countries, but rare elsewhere.

ISO 8601 includes the ISO week date system, a numbering system for weeks - each week begins on a Monday and is associated with the year that contains that week's Thursday (so that if a year starts in a long weekend Friday-Sunday, week number one of the year will start after that). For example, week 1 of 2004 (2004W01) ran from Monday 29 December 2003 to Sunday, 4 January 2004, because its Thursday was 1 January 2004, whereas week 1 of 2005 (2005W01) ran from Monday 3 January 2005 to Sunday 9 January 2005, because its Thursday was 6 January 2005 and so the first Thursday of 2005. The highest week number in a year is either 52 or 53 (it was 53 in the year 2004).

An ISO week-numbering year (also called ISO year informally) has 52 or 53 full weeks. That is 364 or 371 days instead of the usual 365 or 366 days. The extra week is referred to here as a leap week, although ISO 8601 does not use this term. Weeks start with Monday. The first week of a year is the week that contains the first Thursday (and, hence, 4 January) of the year. ISO week year numbering therefore slightly deviates from the Gregorian for some days close to 1 January.


Ordinal Day

If the ordinal date is not known, it can be computed by any of several methods. perhaps the most direct is a table such as the following:

 To the day of:  Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
 Add:            0   31  59  90  120 151 181 212 243 273 304 334
 For leap years: 0   31  60  91  121 152 182 213 244 274 305 335

Week Number

Calculating the week number of a given date

The week number of any date can be calculated, given its ordinal date (i.e. position within the year) and its day of the week.

Method: Using ISO weekday numbers (running from 1 for Monday to 7 for Sunday), subtract the weekday from the ordinal date, then add 10. Divide the result by 7. Ignore the remainder; the quotient equals the week number. If the week number thus obtained equals 0, it means that the given date belongs to the preceding (week-based) year. If a week number of 53 is obtained, one must check that the date is not actually in week 1 of the following year.

    week(date) = int((ordinal(date) - weekday(date) + 10)/7)

Example: Friday 26 September 2008

    Ordinal day: 244 + 26 = 270
    Weekday: Friday = 5
    270 - 5 + 10 = 275
    275 / 7 = 39.28
    Result: Week 39

53 Week Years

The long years, with 53 weeks in them, can be described by any of the following equivalent definitions:

 - any year starting on Thursday and any leap year starting on Wednesday
 - any year ending on Thursday and any leap year ending on Friday
 - years in which 1 January and 31 December (in common years) or either (in leap years) are Thursdays

Day of Week

The tabular forerunner to Tondering's algorithm is embodied in the following ANSI C function. With minor changes, it is adaptable to other high level programming languages such as APL2. (A 6502 assembly language version exists as well.) Devised by Tomohiko Sakamoto in 1993, it is accurate for any Gregorian date:

   int dow(int y, int m, int d)
       static int t[] = {0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4};
       y -= m < 3;
       return (y + y/4 - y/100 + y/400 + t[m-1] + d) % 7;

The function returns 0 = Sunday, 1 = Monday, etc.


Versions prior to 1.5 did not follow ISO 8601.

None, however please contact the author at should you find any problems and I will endevour to resolve then as soon as possible.


 Greg George, IT Technology Solutions P/L, Australia
 Mobile: +61-404-892-159, Email:


Date::Parse or check CPAN


Thanks to Alexandr Ciornii for the V1.3 updates Thanks to Niel Bowers for [ #93599] Not clear what type of week number is returned


Revision 1.7 2020/01/04 Greg - Resolved issue with tests failing. Now pass the full 4 digit year to timelocal() to avoid ambigous date values. - No changes to the actual code

Revision 1.6 2014/04/09 Greg - Allow extended date coverage - past dates handled by localtime - using hash ref argument year, month, day - Added mode for ISO 8601 output YYYY-Wxx

Revision 1.5 2014/03/16 Greg - Updated to conform to ISO 8601 - Added compatability flag to allow backwards usage

Revision 1.4 2009/06/21 07:29:05 Greg - Added ACKNOWLEDGEMENTS

Revision 1.3 2009/06/20 09:31:39 Greg - Real tests with Test::More - Tests moved to t/ - Better Makefile.PL - Now WeekOfYear can take an argument (unixtime)

Revision 1.2 2006/06/11 02:28:55 Greg - Correction to name of function

Revision 2004/08/09 11:07:15 Greg - Initial release to CPAN


$Id:,v 1.4 2009/06/21 07:29:05 Greg Exp $