[PATCH 2/3] Allow Git::get_tz_offset to properly handle DST boundary times
- From
Ben Walton <bdwalton@gmail.com>
- Date
- Jan 15, 2013, 23:10 UTC
- Message-ID
- <1358291405-10173-3-git-send-email-bdwalton@gmail.com>
- In-Reply-To
- <1358291405-10173-1-git-send-email-bdwalton@gmail.com>
The Git::get_tz_offset is meant to provide a workalike replacement for the GNU strftime %z format specifier. The algorithm used failed to properly handle DST boundary cases.
For example, the unix time 1162105199 in CST6CDT saw set_tz_offset improperly return -0600 instead of -0500.
TZ=CST6CDT date -d @1162105199 +"%c %z" Sun 29 Oct 2006 01:59:59 AM CDT -0500
$ zdump -v /usr/share/zoneinfo/CST6CDT | grep 2006 /usr/share/zoneinfo/CST6CDT Sun Apr 2 07:59:59 2006 UTC = Sun Apr 2 01:59:59 2006 CST isdst=0 gmtoff=-21600 /usr/share/zoneinfo/CST6CDT Sun Apr 2 08:00:00 2006 UTC = Sun Apr 2 03:00:00 2006 CDT isdst=1 gmtoff=-18000 /usr/share/zoneinfo/CST6CDT Sun Oct 29 06:59:59 2006 UTC = Sun Oct 29 01:59:59 2006 CDT isdst=1 gmtoff=-18000 /usr/share/zoneinfo/CST6CDT Sun Oct 29 07:00:00 2006 UTC = Sun Oct 29 01:00:00 2006 CST isdst=0 gmtoff=-21600
To determine how many hours/minutes away from GMT a particular time was, we calculated the gmtime() of the requested time value and then used Time::Local's timelocal() function to turn the GMT-based time back into a scalar value representing seconds from the epoch. Because GMT has no daylight savings time, timelocal() cannot handle the ambiguous times that occur at DST boundaries since there are two possible correct results.
To work around the ambiguity at these boundaries, we must take into account the pre and post conversion values for is_dst as provided by both the original time value and the value that has been run through timelocal(). If the is_dst field of the two times disagree then we must modify the value returned from timelocal() by an hour in the correct direction.
Signed-off-by: Ben Walton <bdwalton@gmail.com> --- perl/Git.pm | 20 ++++++++++++++++++++ 1 file changed, 20 insertions(+)
diff --git a/perl/Git.pm b/perl/Git.pm index 5649bcc..788b9b4 100644 --- a/perl/Git.pm +++ b/perl/Git.pm @@ -528,7 +528,27 @@ If TIME is not supplied, the current local time is used. sub get_tz_offset { # some systmes don't handle or mishandle %z, so be creative. my $t = shift || time; + # timelocal() has a problem when it comes to DST ambiguity so + # times that are on a DST boundary cannot be properly converted + # using it. we will possibly adjust its result depending on whehter + # pre and post conversions agree on DST my $gm = timelocal(gmtime($t)); + + # we need to know whether we were originally in DST or not + my $orig_dst = (localtime($t))[8]; + # and also whether timelocal thinks we're in DST + my $conv_dst = (localtime($gm))[8]; + + # re-adjust $gm based on the DST value for the two times we're + # handling. + if ($orig_dst != $conv_dst) { + if ($orig_dst == 1) { + $gm -= 3600; + } else { + $gm += 3600; + } + } + my $sign = qw( + + - )[ $t <=> $gm ]; return sprintf("%s%02d%02d", $sign, (gmtime(abs($t - $gm)))[2,1]); }
-- 1.7.10.4