Strange result of localtime()

nite_man has asked for the wisdom of the Perl Monks concerning the following question:

Hi wise monks!

I've found a very strange result of function localtime(). Let's see simple peace of code:

for my $val ( qw(1 5 10 24) ) {

    print "Add $val hour ...\n";

    for my $date (qw(1096600000 1099170000) ) {
        print "\tBefore: $date - ".localtime($date)."\n";

        print "\tAfter: ".($date+$val*3600)." - ".localtime($date+$val
+*3600)."\n";

    }

    print "\n";

}
[download]

After run it I've got those results:

Add 1 hour ...
        Before: 1096600000 - Fri Oct  1 06:06:40 2004
        After: 1096603600 - Fri Oct  1 07:06:40 2004
        Before: 1099170000 - Sun Oct 31 00:00:00 2004
        After: 1099173600 - Sun Oct 31 01:00:00 2004

Add 5 hour ...
        Before: 1096600000 - Fri Oct  1 06:06:40 2004
        After: 1096618000 - Fri Oct  1 11:06:40 2004
        Before: 1099170000 - Sun Oct 31 00:00:00 2004
        After: 1099188000 - Sun Oct 31 04:00:00 2004

Add 10 hour ...
        Before: 1096600000 - Fri Oct  1 06:06:40 2004
        After: 1096636000 - Fri Oct  1 16:06:40 2004
        Before: 1099170000 - Sun Oct 31 00:00:00 2004
        After: 1099206000 - Sun Oct 31 09:00:00 2004

Add 24 hour ...
        Before: 1096600000 - Fri Oct  1 06:06:40 2004
        After: 1096686400 - Sat Oct  2 06:06:40 2004
        Before: 1099170000 - Sun Oct 31 00:00:00 2004
        After: 1099256400 - Sun Oct 31 23:00:00 2004
[download]

It's amazing! Any ideads why result is "Sun Oct 31 23:00:00 2004"? It should be "Mon Nov 1 00:00:00 2004"!

Well, maybe there is a bug in the implementation of function localtime() but possibly it happened because today is my last work day before holiday :)

---
Michael Stepanov aka nite_man

It's only my opinion and it doesn't have pretensions of absoluteness!

Comment on Strange result of localtime() Select or Download Code

Replies are listed 'Best First'.
Re: Strange result of localtime() by muntfish (Chaplain) on Sep 30, 2004 at 11:29 UTC
According to my calendar, Oct 31 is the end of daylight saving time, a.k.a. summer time. `localtime` is correcting for this extra hour. If you use `gmtime` instead, it won't do that. s^^unp(;75N=&9I<V@`ack(u,^;s\|$.+\`\|"$`$'\"$&\"$"\|ee;/m.+h/&&print$&	[reply] [d/l] [select]
Re^2: Strange result of localtime() by nite_man (Deacon) on Sep 30, 2004 at 12:23 UTC
Yes, you are right, muntfish. I've forgotten about that. Thanks for your replay. --- Michael Stepanov aka nite_man It's only my opinion and it doesn't have pretensions of absoluteness!	[reply]
Re: Strange result of localtime() by Corion (Patriarch) on Sep 30, 2004 at 11:25 UTC
localtime respects daylight savings time, therefore any date math based on that should restricted to adding 22 hours repeatedly until the date changes and then subtracting the fractional part of the day.	[reply]