Calculating the week number for american calendars

traene · 04-22-2005, 01:28 PM

In american calendars the first day in a week is sunday. What week
was started on Dec. 26th 2004. Is it the first week of 2005 or the
last week of 2004?
Can anybody have a look in his local calendar?

I have tried it in Java with the GregorianCalendar and
Locale.US.

And how can this be done with c++?
I tried:

Code:

#include <ctime>
#include <iostream>
using namespace std;

// call with prog YEAR MOTH DATE
int main() {
	if (argc<4) return 0;

	struct tm _tm;
	time_t now = time(NULL);
	gmtime_r( &now, &_tm);
	_tm.tm_year = atoi(argv[1]) - 1900;
	_tm.tm_mon  = atoi(argv[2]) - 1;
	_tm.tm_mday = atoi(argv[3]);
	
	cout << _tm.tm_year << _tm.tm_mon << _tm.tm_mday << endl;
	time_t sec = mktime(&_tm);
	cout << "Seconds: " << sec << "  now: " << now << endl;
	
	char buf[8];
	size_t size = strftime(buf, 8, "%G%V", &_tm);
	
	cout << "Week: " << buf << endl;
}

I started this with LANG=en_US, and it prints

Code:

Week: 200452

Which is correct for european calendars.

homey · 04-22-2005, 02:30 PM

Here's the output of cal 12

Code:

       January               February                 March        
Su Mo Tu We Th Fr Sa   Su Mo Tu We Th Fr Sa   Su Mo Tu We Th Fr Sa 
                1  2       1  2  3  4  5  6          1  2  3  4  5
 3  4  5  6  7  8  9    7  8  9 10 11 12 13    6  7  8  9 10 11 12
10 11 12 13 14 15 16   14 15 16 17 18 19 20   13 14 15 16 17 18 19
17 18 19 20 21 22 23   21 22 23 24 25 26 27   20 21 22 23 24 25 26
24 25 26 27 28 29 30   28 29                  27 28 29 30 31
31
        April                   May                   June         
Su Mo Tu We Th Fr Sa   Su Mo Tu We Th Fr Sa   Su Mo Tu We Th Fr Sa 
                1  2    1  2  3  4  5  6  7             1  2  3  4
 3  4  5  6  7  8  9    8  9 10 11 12 13 14    5  6  7  8  9 10 11
10 11 12 13 14 15 16   15 16 17 18 19 20 21   12 13 14 15 16 17 18
17 18 19 20 21 22 23   22 23 24 25 26 27 28   19 20 21 22 23 24 25
24 25 26 27 28 29 30   29 30 31               26 27 28 29 30

        July                  August                September      
Su Mo Tu We Th Fr Sa   Su Mo Tu We Th Fr Sa   Su Mo Tu We Th Fr Sa 
                1  2       1  2  3  4  5  6                1  2  3
 3  4  5  6  7  8  9    7  8  9 10 11 12 13    4  5  6  7  8  9 10
10 11 12 13 14 15 16   14 15 16 17 18 19 20   11 12 13 14 15 16 17
17 18 19 20 21 22 23   21 22 23 24 25 26 27   18 19 20 21 22 23 24
24 25 26 27 28 29 30   28 29 30 31            25 26 27 28 29 30
31
       October               November               December       
Su Mo Tu We Th Fr Sa   Su Mo Tu We Th Fr Sa   Su Mo Tu We Th Fr Sa 
                   1          1  2  3  4  5                1  2  3
 2  3  4  5  6  7  8    6  7  8  9 10 11 12    4  5  6  7  8  9 10
 9 10 11 12 13 14 15   13 14 15 16 17 18 19   11 12 13 14 15 16 17
16 17 18 19 20 21 22   20 21 22 23 24 25 26   18 19 20 21 22 23 24
23 24 25 26 27 28 29   27 28 29 30            25 26 27 28 29 30 31
30 31

traene · 04-23-2005, 04:29 AM

Thanks for your reply. But what particular week started on Dec 26h 2004.
I guess it should be the last week of 2005.

Code:

       October               November               December
Su Mo Tu We Th Fr Sa   Su Mo Tu We Th Fr Sa   Su Mo Tu We Th Fr Sa
                1  2       1  2  3  4  5  6             1  2  3  4
 3  4  5  6  7  8  9    7  8  9 10 11 12 13    5  6  7  8  9 10 11
10 11 12 13 14 15 16   14 15 16 17 18 19 20   12 13 14 15 16 17 18
17 18 19 20 21 22 23   21 22 23 24 25 26 27   19 20 21 22 23 24 25
24 25 26 27 28 29 30   28 29 30               26 27 28 29 30 31
31

My Java program looks like this:

Code:

	public static void calendarTest(){
		Locale.setDefault(Locale.GERMAN);
		//Locale.setDefault(Locale.UK);
		//Locale.setDefault(Locale.US);
		for (int date=25; date <=27; ++ date) {
			int year = 2004, month = 11;
			Calendar cal = new GregorianCalendar(year, month, date);
			
			DateFormat sdf = SimpleDateFormat.getDateInstance(DateFormat.SHORT);
			DateFormat df = new SimpleDateFormat("yyyyww");
			System.out.println("Date: " + sdf.format(cal.getTime()) 
					+ "  Week:" + df.format(cal.getTime()));
		}
	}

If you try different locales, you get fairly different results. If you try the GERMAN Locale you get:

Code:

Date: 25.12.04  Week:200452
Date: 26.12.04  Week:200452
Date: 27.12.04  Week:200453

The US Locale gives:

Code:

Date: 12/25/04  Week:200452
Date: 12/26/04  Week:200401
Date: 12/27/04  Week:200401

So i wonder if really the first week of 2005 started on Dec, 26th. The UK Locale differs from
the US locale that the week starts on Monday instead of sunday. But It also starts the 'new
year' in december.

I did not check the ISO standard for week calculation for now. But i think the weeks
are defined there as starting on monday.

traene · 04-23-2005, 05:35 AM

I found another interessting thing:
In the GERMAN locale the first year-week 199801
started on Dec, 29th 1997. I guess this is ISO 8601 conform.

I guess my question is (yeah, sounds silly!): What is the
american standard for defining week numbers.
here are
some informations on this. But this:

Quote:

US Internal Revenue Service Weeks

The US IRS evidently clusters weeks into monthly 4/5-week Report Cycles, numbering the weeks as YYYYWW from Jan 1 or near that; details are unknown.

does not help very much.

Thanks for your convenience.