Change in output of strftime from v5.40 to v5.42

[simcop2387]

Module: POSIX

Description
Reported on IRC by sphex, it looks like there's a change in behavior of POSIX::strftime between v5.40..v5.42

sphex: anyone else seeing a different (incorrect) result on 5.42 than on 5.40 with: TZ=Europe/Paris perl -E 'use POSIX "strftime"; say strftime("%FT%T%z", localtime 1761436800);'

Steps to Reproduce
5.42:
perl -E 'BEGIN {$ENV{TZ}="Europe/Paris"} use POSIX "strftime"; say strftime("%FT%T%z", localtime 1761436800);'
2025-10-26T02:00:00+0100

5.40
perl -E 'BEGIN {$ENV{TZ}="Europe/Paris"} use POSIX "strftime"; say strftime("%FT%T%z", localtime 1761436800);'
2025-10-26T02:00:00+0200

Expected behavior
I believe the answer from 5.40 is the correct output.

I bisected this with the following command:

Porting/bisect.pl --start v5.40.0 --end v5.42.0 -e ' BEGIN {$ENV{TZ}="Europe/Paris"} use POSIX "strftime"; die "Wrong time" unless "2025-10-26T02:00:00+0200" eq strftime("%FT%T%z", localtime 1761436800);'

and got: commit 86a9c18 as the culprit
Perl configuration

Summary of my perl5 (revision 5 version 42 subversion 0) configuration:

  Platform:
    osname=linux
    osvers=6.12.22+bpo-amd64
    archname=x86_64-linux
    uname='linux simcop2387.info 6.12.22+bpo-amd64 #1 smp preempt_dynamic debian 6.12.22-1~bpo12+1 (2025-04-25) x86_64 gnulinux '
    config_args='-Dprefix=/home/perlbot/perl5/custom/5.42.0 -de -Dusedevel -Duseshrplib -Dusemymalloc=y -Accflags=-fpie -fPIC -mtune=native -fstack-protector-all -pie -D_FORTIFY_SOURCE=2 -ggdb  -DPERL_EMERGENCY_SBRK -Aldflags=-Wl,-z,now -Wl,-zrelro -Wl,-z,noexecstack -A'eval:scriptdir=/home/perlbot/perl5/custom/5.42.0/bin''
    hint=recommended
    useposix=true
    d_sigaction=define
    useithreads=undef
    usemultiplicity=undef
    use64bitint=define
    use64bitall=define
    uselongdouble=undef
    usemymalloc=y
    default_inc_excludes_dot=define

  Compiler:
    cc='cc'
    ccflags ='-fpie -fPIC -mtune=native -fstack-protector-all -pie -D_FORTIFY_SOURCE=2 -ggdb -DPERL_EMERGENCY_SBRK -fwrapv -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64'
    optimize='-O2'
    cppflags='-fpie -fPIC -mtune=native -fstack-protector-all -pie -D_FORTIFY_SOURCE=2 -ggdb -DPERL_EMERGENCY_SBRK -fwrapv -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include'
    ccversion=''
    gccversion='12.2.0'
    gccosandvers=''
    intsize=4
    longsize=8
    ptrsize=8
    doublesize=8
    byteorder=12345678
    doublekind=3
    d_longlong=define
    longlongsize=8
    d_longdbl=define
    longdblsize=16
    longdblkind=3
    ivtype='long'
    ivsize=8
    nvtype='double'
    nvsize=8
    Off_t='off_t'
    lseeksize=8
    alignbytes=8
    prototype=define
  Linker and Libraries:
    ld='cc'
    ldflags =' -Wl,-z,now -Wl,-zrelro -Wl,-z,noexecstack -fstack-protector-strong -L/usr/local/lib'
    libpth=/usr/local/lib /usr/lib/x86_64-linux-gnu /usr/lib /usr/lib64
    libs=-lpthread -ldl -lm -lcrypt -lutil -lc
    perllibs=-lpthread -ldl -lm -lcrypt -lutil -lc
    libc=/lib/x86_64-linux-gnu/libc.so.6
    so=so
    useshrplib=true
    libperl=libperl.so
    gnulibc_version='2.36'
  Dynamic Linking:
    dlsrc=dl_dlopen.xs
    dlext=so
    d_dlsymun=undef
    ccdlflags='-Wl,-E -Wl,-rpath,/home/perlbot/perl5/custom/5.42.0/lib/5.42.0/x86_64-linux/CORE'
    cccdlflags='-fPIC'
    lddlflags='-shared -O2 -L/usr/local/lib -fstack-protector-strong'

Characteristics of this binary (from libperl): 
  Compile-time options:
    HAS_LONG_DOUBLE
    HAS_STRTOLD
    HAS_TIMES
    MYMALLOC
    PERLIO_LAYERS
    PERL_COPY_ON_WRITE
    PERL_DONT_CREATE_GVSV
    PERL_HASH_FUNC_SIPHASH13
    PERL_HASH_USE_SBOX32
    PERL_MALLOC_WRAP
    PERL_OP_PARENT
    PERL_PRESERVE_IVUV
    PERL_USE_DEVEL
    PERL_USE_SAFE_PUTENV
    USE_64_BIT_ALL
    USE_64_BIT_INT
    USE_LARGE_FILES
    USE_LOCALE
    USE_LOCALE_COLLATE
    USE_LOCALE_CTYPE
    USE_LOCALE_NUMERIC
    USE_LOCALE_TIME
    USE_PERLIO
    USE_PERL_ATOF
  Built under linux
  Compiled at Jul  3 2025 21:14:07
  %ENV:
    PERLBREW_HOME="/home/perlbot/.perlbrew"
    PERLBREW_MANPATH="/home/perlbot/perl5/perlbrew/perls/perlbot-main/man"
    PERLBREW_PATH="/home/perlbot/perl5/perlbrew/bin:/home/perlbot/perl5/perlbrew/perls/perlbot-main/bin"
    PERLBREW_PERL="perlbot-main"
    PERLBREW_ROOT="/home/perlbot/perl5/perlbrew"
    PERLBREW_SHELLRC_VERSION="0.86"
    PERLBREW_VERSION="0.86"
  @INC:
    /home/perlbot/perl5/custom/5.42.0/lib/site_perl/5.42.0/x86_64-linux
    /home/perlbot/perl5/custom/5.42.0/lib/site_perl/5.42.0
    /home/perlbot/perl5/custom/5.42.0/lib/5.42.0/x86_64-linux
    /home/perlbot/perl5/custom/5.42.0/lib/5.42.0

[jkeenan]

Module: POSIX

I bisected this with the following command:

Porting/bisect.pl --start v5.40.0 --end v5.42.0 -e ' BEGIN {$ENV{TZ}="Europe/Paris"} use POSIX "strftime"; die "Wrong time" unless "2025-10-26T02:00:00+0200" eq strftime("%FT%T%z", localtime 1761436800);'

and got: commit 86a9c18 as the culprit Perl configuration

@khwilliamson, that was your commit; can you investigate? Thanks.

[khwilliamson]

Note #22351

[jkeenan]

Module: POSIX

Description Reported on IRC by sphex, it looks like there's a change in behavior of POSIX::strftime between v5.40..v5.42

Yes, there was.

sphex: anyone else seeing a different (incorrect) result on 5.42 than on 5.40 with: TZ=Europe/Paris perl -E 'use POSIX "strftime"; say strftime("%FT%T%z", localtime 1761436800);'

Steps to Reproduce 5.42: perl -E 'BEGIN {$ENV{TZ}="Europe/Paris"} use POSIX "strftime"; say strftime("%FT%T%z", localtime 1761436800);' 2025-10-26T02:00:00+0100

5.40 perl -E 'BEGIN {$ENV{TZ}="Europe/Paris"} use POSIX "strftime"; say strftime("%FT%T%z", localtime 1761436800);' 2025-10-26T02:00:00+0200

Expected behavior I believe the answer from 5.40 is the correct output.

No, we believe that that output is incorrect; the output from 5.42 is correct.

See @khwilliamson's explanation in 4c99243,

[simcop2387]

What's happening is that the time zone is changing an hour early, instead of at 03:00 when it's supposed to ( https://en.wikipedia.org/wiki/Time_in_France#Summer_time ). 5.40 follows this correctly, 5.42 does not.

I expanded the test case to demonstrate better

BEGIN {$ENV{TZ}="Europe/Paris"} use POSIX "strftime"; [map {strftime("%F %T%z", localtime 1761436800+$_)} -1,0,1,3599,3600,3601,7200,7201]

[
"2025-10-26 01:59:59+0200", # -1 second
"2025-10-26 02:00:00+0100", # +0 seconds
"2025-10-26 02:00:01+0100", # +1 second
"2025-10-26 02:59:59+0100", # +59 minutes 59 seconds
"2025-10-26 02:00:00+0100", # +1 hour, repeating 02:00+0100?
"2025-10-26 02:00:01+0100", # +1 hour 1 second
"2025-10-26 03:00:00+0100", # +2 hours
"2025-10-26 03:00:01+0100"  # +2 hours 1 second
]

5.40: same code

[
"2025-10-26 01:59:59+0200", # -1 second
"2025-10-26 02:00:00+0200", # +0 seconds
"2025-10-26 02:00:01+0200", # +1 second
"2025-10-26 02:59:59+0200", # +59 minutes 59 seconds
"2025-10-26 02:00:00+0100", # +1 hour, 
"2025-10-26 02:00:01+0100", # +1 hour 1 second
"2025-10-26 03:00:00+0100", # +2 hours
"2025-10-26 03:00:01+0100"  # +2 hours 1 second
]

The output from 5.42 does not look right at all to me with this