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By: Boris Gasin,
Vice President of Technology,
Dynamic Technology Group

Question: How do I calculate the week number of a specific date?

Answer: There are a few different date standards, and each can affect how the week number is calculated. Before attempting to calculate a week number, you should resolve the following questions:

What is the first day of the week?

When does the first week start? In the United States, it is accepted that the week begins on a Sunday. This convention has deep religious roots and goes back to the book of Genesis.

However, the prevailing international standard for date and time formats is ISO 8601. According to ISO 8601, Monday is considered to be the first day of the week, and the first week of the year is the first week that has four days in the new year. A complete standard may be downloaded from the ISO Web site at http://www.iso.ch/markete/8601.pdf. Other conventions may be based on specific business rules.

The functions below provide an example of the week number calculation for the ISO standard. The algorithm can be easily modified to meet the requirements of a different convention.

Because the standard Powerscript DayNumber() function represents the US standard, a more generic of_daynumber function is created.

// Method: of_daynumber
// Author: B. Gasin
// Date  : 11/10/99
// Arg  :
// Return: integer
// Desc  : Function returns number of the
// day in the week
//	    ISO    US	
//    Mon = 1	    2
//    Tue = 2	    3
//    Wed = 3	    4
//    Thu = 4	    5
//    Fri = 5	    6
//    Sat = 6	    7
//    Sun = 7	    1
// Modifications:
// Date     Author              Comments
integer li_rc
li_rc = DayNumber( ad_date )
// Convert to ISO
IF of_get_mode() = ISO then 
// ISO is a constant
li_rc --
IF li_rc = 0 THEN li_rc = 7
RETURN li_rc
Because the algorithm for the week number calculation is nonlinear, it has proved to be a little tricky. Up to first three days in January may belong to a previous year. Some years may contain 52 weeks, while others may have 53.

To determine the week number, I went with the "brute force" approach. First I calculated a starting point?a Thursday either preceding or following the date in question. Next I counted the weeks backward until the beginning of the year, as shown below.

// Method: of_isoweeknumber
// Author: B. Gasin
// Date : 11/10/99
// Arg : ad_date
// Return: integer
// Desc : Determines the week number of 
//        a date according to ISO
//	  standard.
// ISO Rules:
// - Monday is a 1st day of the week
// - 1st week of the year is considered 
//    to be the first week with 4 days
// Modifications:
// Date Author Comments

date ld_date
Integer li_year, li_prev_year
Integer li_day, li_week

ld_date = ad_date

li_day = of_DayNumber(ld_date)

DO WHILE li_day <> 4 
IF li_day < 4 THEN 
   // Mon - Wed. Go to next Thur.
   ld_date = RelativeDate(ld_date, 1)
   // Thur - Sun. Go to prev Thur.
   ld_date = RelativeDate(ld_date, -1)
li_day = of_DayOfWeek(ld_date)

li_year = Year(ld_date)

// Count weeks backwards until the
// previous year
   ld_date = RelativeDate(ld_date, -7)
   li_prev_year = Year(ld_date)
LOOP UNTIL li_year <> li_prev_year

Return li_week

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