How To Get ISO-8601 GMT/UTC Week Number of Year

Question

In Objective C, how do I take the GMT/UTC date and apply the ISO-8601 rule to get the week number?

PHP generates a week number of the year in ISO-8601 format when one uses the gmdate('W') statement. This, however, doesn't match in Objective C when you try to just take the GMT/UTC date and get the week number because it doesn't do it in the ISO-8601 way. Here's what the PHP documentation says about ISO-8601 with the 'W' parameter:

"W: ISO-8601 week number of year, weeks starting on Monday" [emphasis mine]

So, when I look at my calendar for 2016, Jan 26 falls on week 05 if I disregard that "starting on Monday" rule, and falls on 04 if I take that rule into consideration.

Compare these two examples, one in PHP, the other in Objective C, and you'll get two different results:

Objective C

NSCalendar *calender = [NSCalendar currentCalendar];
NSDateComponents *dateComponent = [calender components:(NSWeekOfYearCalendarUnit | NSDayCalendarUnit | NSMonthCalendarUnit | NSYearCalendarUnit) fromDate:[NSDate date]];
NSString *sWeekNum = [NSString stringWithFormat:@"%ld",(long)dateComponent.weekOfYear];
if ([sWeekNum length] < 2) {
  sWeekNum = [NSString stringWithFormat:@"0%ld",(long)dateComponent.weekOfYear];
}
NSLog(@"%@",sWeekNum);

PHP

<?php
error_reporting(E_ALL);
ini_set('display_errors','On');

// SET OUR TIMEZONE STUFF
try {
    $sTimeZone = 'GMT';
    if (function_exists('date_default_timezone_set')) {
        date_default_timezone_set($sTimeZone);
    } else {
        putenv('TZ=' .$sTimeZone);
    }
    ini_set('date.timezone', $sTimeZone);
} catch(Exception $e) {}
echo gmdate('W') . "\n";

For instance, today is Jan 26, 2016 @ 1:48am EST for me. The Objective C emits 05, while the PHP emits 04.

score 2 · Accepted Answer · edited Jan 26 '16 at 08:48

2

NSCalendar can be initialized as ISO8601 calendar.

NSCalendar *calender = [[NSCalendar alloc] initWithCalendarIdentifier:NSCalendarIdentifierISO8601];
unsigned int weekOfYear = (unsigned int)[calender component:NSCalendarUnitWeekOfYear fromDate: [NSDate date]];
NSString *sWeekNum = [NSString stringWithFormat:@"%02u",weekOfYear];
NSLog(@"%@", sWeekNum);

edited Jan 26 '16 at 08:48

Volomike

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answered Jan 26 '16 at 08:23

vadian

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I had to switch NSInteger to a long, but other than that it worked. Do you know how to use NSDateFormatter to use %W to make the two digit version? – Volomike Jan 26 '16 at 08:26
Thanks a bunch. When I get clearance, I'll mark this as the best answer. – Volomike Jan 26 '16 at 08:34
@Volomike If you have edited my answer please use the proper format specifier (`unsigned int` is `%u`). – vadian Jan 26 '16 at 08:45
Ah, didn't realize that. I'll do that. – Volomike Jan 26 '16 at 08:48

Volomike · Answer 2 · 2016-01-26T08:12:39.030

I couldn't figure out the solution in Objective C completely. I had to switch to C/C++ (C in this case), which required that I changed my .m file (where I was building the code) into a .mm file so that I could mix C/C++ with Objective C, and then ensure that this was compiled in the project settings.

#include <string>
#include <time.h>
//...and then, later on in the code...
time_t rawtime;
struct tm *t;
char b[3]; // 2 chars + \0 for C strings
time( &rawtime );
t = gmtime(&rawtime);
strftime(b,3,"%W",t);
NSString *sWeekNum = [NSString stringWithFormat:@"%s",b];

This also ensures it's 2 digits. So, one doesn't have to do that extra step in order to make it match the PHP version.

How To Get ISO-8601 GMT/UTC Week Number of Year

Objective C

PHP

2 Answers2