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RE: FW: timelocal() discrepancy vs DateTime->epoch

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From:
Lu Feng
Date:
March 16, 2017 14:06
Subject:
RE: FW: timelocal() discrepancy vs DateTime->epoch
Message ID:
20170316140643.DCEE011D402@xx1.develooper.com
Hi Bill,

You are right that the problem I noticed with timelocal() around 11/6/1910
is only present on Windows.
On my Linux host, timelocal() remains consistent with DateTime module around
11/6/1910.

But your prediction for Apr 2 in 1906 does not pan out on Windows.

G:\>perl David.pl 1906 04 01 08 00 00
Epoch for 19060401:080000 (UTC) =       -2011881600     based on Epoch().
Epoch for 19060401:080000 (UTC) =       -2011881600     based on timegm().
Epoch for 19060401:080000 (NYC) =       -2011863600     based on Epoch().
Epoch for 19060401:080000 (NYC) =       -2011867200     based on
timelocal().

G:\>perl David.pl 1906 04 02 08 00 00
Epoch for 19060402:080000 (UTC) =       -2011795200     based on Epoch().
Epoch for 19060402:080000 (UTC) =       -2011795200     based on timegm().
Epoch for 19060402:080000 (NYC) =       -2011777200     based on Epoch().
Epoch for 19060402:080000 (NYC) =       -2011780800     based on
timelocal().

G:\>perl David.pl 1906 04 03 08 00 00
Epoch for 19060403:080000 (UTC) =       -2011708800     based on Epoch().
Epoch for 19060403:080000 (UTC) =       -2011708800     based on timegm().
Epoch for 19060403:080000 (NYC) =       -2011690800     based on Epoch().
Epoch for 19060403:080000 (NYC) =       -2011694400     based on
timelocal().

timelocal() assigns UNIX times -2011867200, -2011780800 and -2011694400
respectively, exactly 86400 apart and thus shows no sign of DST switch on
either 19060402 or 19060403.

----------------------------------------------------------------------------
-----------------------

As Dave mentioned, that the concept of local time in USA before 11/18/1883
(when railroad finally brought in the standard time) was all but
meaningless, so I would just retrict timelocal() and localtime() usage after
11/19/1883 on Linux (and after 11/7/1910 on Windows).

But a more intriguing problem I noticed is that, on both Windows and Linux,
timegm() starts to fail at around 
Feb, year 0000 AD, while gmtime() and DateTime module continue to hold up.

For example:  timegm(gmtime(-62163198000)) returns -62163111600.  The fault
here seems to be with timegm(), not gmtime().

Lu




 

-----Original Message-----
From: Bill Ricker [mailto:bill.n1vux@gmail.com] 
Sent: Thursday, March 16, 2017 7:47 AM
To: Dave Rolsky
Cc: Lu Feng; datetime@perl.org
Subject: Re: FW: timelocal() discrepancy vs DateTime->epoch

> I'm pretty sure timelocal() is wrong here, though I'm not sure why. 
> There was no DST transition in that zone in 1910. In fact, there 
> wasn't really a time zone until 1918. The pre-1918 zone file in the 
> Olson database is just using local solar time, but it's not very 
> meaningful. If you really care about local times in the early 1900s 
> you probably need to calculate the solar noon for the given lat/long.

This is likely to be a Y2K/19100 window wrap issue.

The little dst.pl script that I use to check Olson DB -- used in 2007 to
diagnose which servers needed Olson/TZ DB update -- also reports DST
transitions for 2010 when asked for 1910.
[ i think i grabbed it from blogs.perl.org and enhanced it ? no authorship
is noted ]

$ dst.pl -y 1910
Sun Mar 14 01:59:59 EST 2010 --> Sun Mar 14 03:00:00 EDT 2010 Sun Nov 07
01:59:59 EDT 2010 --> Sun Nov 07 01:00:00 EST 2010

(That script uses localtime(), Time::Local and POSIX qw(strftime), but not
DateTime.)

Supporting evidence -- Early November DST transition was novel in 2007.
(Giving the little trick-or-treaters the extra hour of daylight at
Hallowe'en was obviously desirable for decades, but it took until 2007 to
get it done.)

I predict Lu's script will report DST transitions morning of Apr 2 and Oct
29 in 1906, since those were the dates in 2006.

--
Bill Ricker
bill.n1vux@gmail.com
https://www.linkedin.com/in/n1vux

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