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

X: I want to write timestamps to a text file in a reasonably human-friendly format and then read them back, getting the same epoch second for comparison with localtime.

Y: my first attempt with Time::Piece:

use strict;
use warnings;
use Time::Piece;
use feature 'say';

my $fmt = '%d %b %Y %T %Z';
say localtime->strftime;
say localtime->strftime($fmt);
my $str = '03 Sep 2022 16:10:17 GMT Daylight Time';
my $strtime = Time::Piece->strptime($str, $fmt);
say localtime->epoch;
say $strtime->epoch;

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This returns:

Sat, 03 Sep 2022 16:10:35 GMT Daylight Time
03 Sep 2022 16:10:35 GMT Daylight Time
Garbage at end of string in strptime:  Daylight Time at C:/Strawberry/
+perl/lib/Time/Piece.pm line 598.
Perhaps a format flag did not match the actual input? at C:/Strawberry
+/perl/lib/Time/Piece.pm line 598.
1662217835
1662221417

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This is out by the timezone offset. The docs tell me to look at https://www.unix.com/man-page/FreeBSD/3/strftime/, which says that %z will be treated as a numerical offset. For me, this gives the same as upper case Z, although I can't rule out the influence of Windows on this. DateTime says it won't parse dates and gives no indication I could see of how to handle localtime.

I'd be happy with UTC, but Time::Piece does not seem to support it, which is why I think I may have an XY problem. I can't believe that no-one has invented this wheel, but my searches have been fruitless and all the wheels I have tried seem square. I know that clocks moving back at the end of next month will bite me. All help gratefully received.

Regards,

John Davies

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Re: Reading a timestamp and getting the written value
by haukex (Archbishop) on Sep 03, 2022 at 15:51 UTC
    my $fmt = '%d %b %Y %T %Z';

    Don't use %Z as it may produce ambiguous time zone names like CST ("Central Standard Time", "China Standard Time", "Cuba Standard Time", ...), and as you can tell Time::Piece's strptime can't handle parsing its own output. Personally, I usually use DateTime::Format::Strptime for parsing, but AFAICT Time::Piece works with %z as well.

    use warnings;
use strict;
use Time::Piece;

my $fmt = '%d %b %Y %T %z';  # %z instead of %Z appears to work
my $str = localtime->strftime($fmt);
print $str, "\n";
my $strtime = Time::Piece->strptime($str, $fmt);
print $strtime->strftime($fmt), "\n";

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    use warnings;
use strict;
use DateTime;
use DateTime::Format::Strptime;

my $fmt = '%d %b %Y %T %z';

my $strp = DateTime::Format::Strptime->new( on_error=>'croak',
    pattern=>$fmt );

my $dt = DateTime->now;
$dt->set_time_zone('America/Chicago');  # try this with "%Z"...
print $dt->rfc3339, "\n";
my $str = $dt->strftime($fmt);
print "$str\n";
my $dt2 = $strp->parse_datetime($str);
print $dt2->rfc3339, "\n";

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[select]

      Thank you; your DateTime incantations work perfectly. I was aware of the issue of ambiguous time zones, but was hoping the longer form output by Time::Piece might work. I can confirm that I have retried the capitalisation of "z" and Time::Piece remains insensitive while DateTime works as expected. It may be a function of Windows 2019 server or Strawberry (This is perl 5, version 32, subversion 1 (v5.32.1) built for MSWin32-x64-multi-thread), but with your fix I'm not investigating further.

      Regards,

      John Davies

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