Re: [PATCH] Call timegm and timelocal with 4-digit year

[Bernhard M. Wiedemann]

On 2018-02-24 23:28, Ævar Arnfjörð Bjarmason wrote:
> My Time::Local 1.2300 on perl 5.024001 currently interprets "69" here as
> 1969, but after this it'll be 2069.

on one hand, there is already
perl -e 'use Time::Local;
  print scalar gmtime(Time::Local::timegm(0,0,0,1,0,68))'
Sun Jan  1 00:00:00 2068

The problem here is the 'currently', because in 11 months from now, 69
will be interpreted as 2069 by Time::Local, too.

faketime 2019-01-01 perl -e 'use Time::Local;
   print scalar gmtime(Time::Local::timegm(0,0,0,1,0,69))'
Tue Jan  1 00:00:00 2069


We could compromize for
$y+=100 if $y<69;

to freeze the behaviour to what Time::Local does in 2018, but then there
might not be actual repos with such 2-digit year timestamps.

There are definitely CVS repos from before 1999 and without this change,
those might be misinterpreted in 2049 (or before, depending on age)

[PATCH] Call timegm and timelocal with 4-digit year

[Bernhard M. Wiedemann]

amazingly timegm(gmtime(0)) is only 0 before 2020
because perl's timegm deviates from GNU timegm(3) in how it handles years.

man Time::Local says

 Whenever possible, use an absolute four digit year instead.

with a detailed explanation about ambiguity of 2-digit years above that.

Even though this ambiguity is error-prone with >50% of users getting it
wrong, it has been like this for 20+ years, so we just use 4-digit years
everywhere to be on the safe side.

We add some extra logic to cvsimport because it allows 2-digit year
input and interpreting an 18 as 1918 can be avoided easily and safely.

Signed-off-by: Bernhard M. Wiedemann <bwiedem...@suse.de>
---
 contrib/examples/git-svnimport.perl | 2 +-
 git-cvsimport.perl  | 4 +++-
 perl/Git.pm | 4 +++-
 perl/Git/SVN.pm | 2 +-
 4 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/contrib/examples/git-svnimport.perl 
b/contrib/examples/git-svnimport.perl
index c414f0d9c..75a43e23b 100755
--- a/contrib/examples/git-svnimport.perl
+++ b/contrib/examples/git-svnimport.perl
@@ -238,7 +238,7 @@ sub pdate($) {
my($d) = @_;
$d =~ m#(\d\d\d\d)-(\d\d)-(\d\d)T(\d\d):(\d\d):(\d\d)#
or die "Unparseable date: $d\n";
-   my $y=$1; $y-=1900 if $y>1900;
+   my $y=$1; $y+=1900 if $y<1000;
return timegm($6||0,$5,$4,$3,$2-1,$y);
 }
 
diff --git a/git-cvsimport.perl b/git-cvsimport.perl
index 2d8df8317..b31613cb8 100755
--- a/git-cvsimport.perl
+++ b/git-cvsimport.perl
@@ -601,7 +601,9 @@ sub pdate($) {
my ($d) = @_;
m#(\d{2,4})/(\d\d)/(\d\d)\s(\d\d):(\d\d)(?::(\d\d))?#
or die "Unparseable date: $d\n";
-   my $y=$1; $y-=1900 if $y>1900;
+   my $y=$1;
+   $y+=100 if $y<70;
+   $y+=1900 if $y<1000;
return timegm($6||0,$5,$4,$3,$2-1,$y);
 }
 
diff --git a/perl/Git.pm b/perl/Git.pm
index ffa09ace9..df62518c7 100644
--- a/perl/Git.pm
+++ b/perl/Git.pm
@@ -534,7 +534,9 @@ If TIME is not supplied, the current local time is used.
 sub get_tz_offset {
# some systems don't handle or mishandle %z, so be creative.
my $t = shift || time;
-   my $gm = timegm(localtime($t));
+   my @t = localtime($t);
+   $t[5] += 1900;
+   my $gm = timegm(@t);
my $sign = qw( + + - )[ $gm <=> $t ];
return sprintf("%s%02d%02d", $sign, (gmtime(abs($t - $gm)))[2,1]);
 }
diff --git a/perl/Git/SVN.pm b/perl/Git/SVN.pm
index bc4eed3d7..991a5885e 100644
--- a/perl/Git/SVN.pm
+++ b/perl/Git/SVN.pm
@@ -1405,7 +1405,7 @@ sub parse_svn_date {
$ENV{TZ} = 'UTC';
 
my $epoch_in_UTC =
-   Time::Local::timelocal($S, $M, $H, $d, $m - 1, $Y - 1900);
+   Time::Local::timelocal($S, $M, $H, $d, $m - 1, $Y);
 
# Determine our local timezone (including DST) at the
# time of $epoch_in_UTC.  $Git::SVN::Log::TZ stored the
-- 
2.13.6