week number of strftime("%U") and %W wrong

[tiran]

Emscripten's strftime() has an off-by-one error for week number with Sunday as first day of a week.

Version of emscripten/emsdk:

emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 3.1.1 (1934a98e709b57d3592b8272d3f1264a72c089e4)
clang version 14.0.0 (https://github.com/llvm/llvm-project f142c45f1e494f8dbdcc1bcf14122d128ac8f3fe)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: /emsdk/upstream/bin

reproducer

#include <time.h>
#include <stdio.h>
#include <stdlib.h>

int
main(void)
{
    char buf[256];
    char *fmt1 = "%Y-%m-%d %H:%M:%S";
    char *fmt2 = "%Y %U %w (%a)";
    time_t t = 1358035200;
    struct tm *tm;

    tm = gmtime(&t);

    strftime(buf, sizeof(buf), fmt1, tm);
    fprintf(stdout, "%s -> %s\n", fmt1, buf);
    strftime(buf, sizeof(buf), fmt2, tm);
    fprintf(stdout, "%s -> %s\n", fmt2, buf);
    return 0;
}

output

$ gcc -o wdbug wdbug.c && ./wdbug 
%Y-%m-%d %H:%M:%S -> 2013-01-13 00:00:00
%Y %U %w (%a) -> 2013 02 0 (Sun)
$ emcc -o wdbug.js wdbug.c && node wdbug.js
%Y-%m-%d %H:%M:%S -> 2013-01-13 00:00:00
%Y %U %w (%a) -> 2013 01 0 (Sun)

python output

# Linux
>>> for day in range(1, 15):
...     print(datetime.datetime(2013, 1, day).strftime("%Y %U %w (%a)"))
... 
2013 00 2 (Tue)
2013 00 3 (Wed)
2013 00 4 (Thu)
2013 00 5 (Fri)
2013 00 6 (Sat)
2013 01 0 (Sun)
2013 01 1 (Mon)
2013 01 2 (Tue)
2013 01 3 (Wed)
2013 01 4 (Thu)
2013 01 5 (Fri)
2013 01 6 (Sat)
2013 02 0 (Sun)
2013 02 1 (Mon)

# wasm32-emscripten
>>> for day in range(1, 15):
...     print(datetime.datetime(2013, 1, day).strftime("%Y %U %w (%a)"))
... 
2013 00 2 (Tue)
2013 00 3 (Wed)
2013 00 4 (Thu)
2013 00 5 (Fri)
2013 00 6 (Sat)
2013 00 0 (Sun)
2013 01 1 (Mon)
2013 01 2 (Tue)
2013 01 3 (Wed)
2013 01 4 (Thu)
2013 01 5 (Fri)
2013 01 6 (Sat)
2013 01 0 (Sun)
2013 02 1 (Mon)

[tiran]

The weeknumber for %W is wrong, too.

#include <time.h>
#include <stdio.h>
#include <stdlib.h>

int
main(void)
{
    char buf[256];
    char *fmt1 = "%Y-%m-%d - %a %w - U:%U W:%W";
    time_t start = 1357041600;
    struct tm *tm;

    for (int i = 0; i < 31; i++) {
        time_t t = start + (i * 60 * 60 * 24);
        struct tm *tm = gmtime(&t);
        strftime(buf, sizeof(buf), fmt1, tm);
        fprintf(stdout, "%s\n", buf);
        if (tm->tm_wday == 0) {
            fprintf(stdout, "---\n");
        }
    }
    return 0;
}

$ emcc -o wdbug.js wdbug.c && node wdbug.js 
2013-01-01 - Tue 2 - U:00 W:00
2013-01-02 - Wed 3 - U:00 W:00
2013-01-03 - Thu 4 - U:00 W:01
2013-01-04 - Fri 5 - U:00 W:01
2013-01-05 - Sat 6 - U:00 W:01
2013-01-06 - Sun 0 - U:00 W:01
---
2013-01-07 - Mon 1 - U:01 W:01
2013-01-08 - Tue 2 - U:01 W:01
2013-01-09 - Wed 3 - U:01 W:01
2013-01-10 - Thu 4 - U:01 W:02
2013-01-11 - Fri 5 - U:01 W:02
2013-01-12 - Sat 6 - U:01 W:02
2013-01-13 - Sun 0 - U:01 W:02
---
2013-01-14 - Mon 1 - U:02 W:02
2013-01-15 - Tue 2 - U:02 W:02
2013-01-16 - Wed 3 - U:02 W:02
2013-01-17 - Thu 4 - U:02 W:03
2013-01-18 - Fri 5 - U:02 W:03
2013-01-19 - Sat 6 - U:02 W:03
2013-01-20 - Sun 0 - U:02 W:03
---
2013-01-21 - Mon 1 - U:03 W:03
2013-01-22 - Tue 2 - U:03 W:03
2013-01-23 - Wed 3 - U:03 W:03
2013-01-24 - Thu 4 - U:03 W:04
2013-01-25 - Fri 5 - U:03 W:04
2013-01-26 - Sat 6 - U:03 W:04
2013-01-27 - Sun 0 - U:03 W:04
---
2013-01-28 - Mon 1 - U:04 W:04
2013-01-29 - Tue 2 - U:04 W:04
2013-01-30 - Wed 3 - U:04 W:04
2013-01-31 - Thu 4 - U:04 W:05

$ gcc -o wdbug wdbug.c && ./wdbug
2013-01-01 - Tue 2 - U:00 W:00
2013-01-02 - Wed 3 - U:00 W:00
2013-01-03 - Thu 4 - U:00 W:00
2013-01-04 - Fri 5 - U:00 W:00
2013-01-05 - Sat 6 - U:00 W:00
2013-01-06 - Sun 0 - U:01 W:00
---
2013-01-07 - Mon 1 - U:01 W:01
2013-01-08 - Tue 2 - U:01 W:01
2013-01-09 - Wed 3 - U:01 W:01
2013-01-10 - Thu 4 - U:01 W:01
2013-01-11 - Fri 5 - U:01 W:01
2013-01-12 - Sat 6 - U:01 W:01
2013-01-13 - Sun 0 - U:02 W:01
---
2013-01-14 - Mon 1 - U:02 W:02
2013-01-15 - Tue 2 - U:02 W:02
2013-01-16 - Wed 3 - U:02 W:02
2013-01-17 - Thu 4 - U:02 W:02
2013-01-18 - Fri 5 - U:02 W:02
2013-01-19 - Sat 6 - U:02 W:02
2013-01-20 - Sun 0 - U:03 W:02
---
2013-01-21 - Mon 1 - U:03 W:03
2013-01-22 - Tue 2 - U:03 W:03
2013-01-23 - Wed 3 - U:03 W:03
2013-01-24 - Thu 4 - U:03 W:03
2013-01-25 - Fri 5 - U:03 W:03
2013-01-26 - Sat 6 - U:03 W:03
2013-01-27 - Sun 0 - U:04 W:03
---
2013-01-28 - Mon 1 - U:04 W:04
2013-01-29 - Tue 2 - U:04 W:04
2013-01-30 - Wed 3 - U:04 W:04
2013-01-31 - Thu 4 - U:04 W:04