I am also curious as to what happens with these ambiguous local times, like in the US 1:23 AM can occur twice during the same day.

What...? Anyway, see the %p and %P tokens.

Invalid offset: CST

As per DateTime::Format::Strptime: "By default, the parser will die when it parses an ambiguous abbreviation."

$ perl -wMstrict -MDateTime -le 'print DateTime->now(time_zone=>$_)->s
+trftime("%Z %z") for qw# America/Havana Asia/Shanghai America/Chicago
+ #'
CST -0500
CST +0800
CST -0600
[download]

See also List of time zone abbreviations, ISO8601 Time zone designators, and Names of time zones.

Also occasionally a specific location can change its time zone - the International Date Line was moved some years back - this affected some islands in the Pacific.

Hence the existence of location-based time zone names that are unambiguous in that respect.

$ perl -wMstrict -MDateTime -le 'print DateTime->new(year=>$_,time_zon
+e=>"Pacific/Apia")->strftime("%Y %z") for qw/ 2010 2012 /'
2010 -1100
2012 +1400
[download]

What...?

Daylight Saving.

---

🦛

Daylight Saving.

Hm, yes, true if one reads the question without the context of the rest of the thread. My confusion stemsmed from the fact that the thread includes YMD and time zone names that would normally resolve that ambiguity. Update: Of course the time zone names used in the following are still ambiguous in other ways, as per my post above. Update 2&3: Upon rereading: the "in the US" bit is still confusing me, since this ambiguity is true of any time zone that observes DST, hence my assumption that this had something to do with AM/PM rather than DST. Other minor reformatting.

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

my $strp = DateTime::Format::Strptime->new(
    pattern => '%F %H:%M %P %Z', on_error => 'croak',
    zone_map => { CST => '-0600', CDT => '-0500' } );

for my $str ('2020-03-08 1:23 AM CST','2020-03-08 1:23 AM CDT') {
    print $str, " = ";
    my $dt = $strp->parse_datetime($str);
    $dt->set_time_zone('UTC');
    print $dt->strftime('%F %T %Z'), " = ";
    $dt->set_time_zone('America/Chicago');
    print $dt->strftime('%F %T %Z'), "\n";
}

__END__

2020-03-08 1:23 AM CST = 2020-03-08 07:23:00 UTC = 2020-03-08 01:23:00
+ CST
2020-03-08 1:23 AM CDT = 2020-03-08 06:23:00 UTC = 2020-03-08 00:23:00
+ CST
[download]

I guess that what this nets out to, is that if you log a "local time", you also need the time_zone.

I am in the US Pacific Time Zone (PST).
Once per year, a "day" will have 25 hours.
My wristwatch will wind up showing 1:15 AM twice during that day.
It is Ok that 1:15 AM PST and 1:15 AM PDST converts to a different GMT time,
but the time zone matters.

I recommend converting all times to GMT/UTC within the DB.

It's too late to be poking at things but I think the problem is that the argument validation is trying to coerce "CST" into a DateTime::TimeZone before it's done anything to do looking at your mapping. I think what that argument is for is providing a mapping for zones if they're named in the date which is being parsed. Tweaking your code slightly:

#!/usr/bin/env perl
use strict;
use warnings;
use 5.010;
use DateTime;
use DateTime::Format::Strptime;
$|=1;

my $str = 'Wed, 13 Jan 2021 17:22:23 CST';
my $pattern = '%a, %d %b %Y %T %Z';
my $strp = DateTime::Format::Strptime->new(
       pattern  => $pattern,
       zone_map => { CST => '-0700', EST => '-0600' }
       );
say "I got this far (not!)";
my $dt = $strp->parse_datetime( $str );
say "String:   $str";
say "DateTime: $dt";
say "zone: ", $dt->time_zone->name;
say '';

__END__
$ perl tz_thing.plx
I got this far (not!)
String:   Wed, 13 Jan 2021 17:22:23 CST
DateTime: 2021-01-13T17:22:23
zone: -0700
[download]

I think what that argument is for is providing a mapping for zones if they're named in the date which is being parsed.

Yes, I agree, time_zone makes sense when the string being parsed doesn't contain a time zone specification, and zone_map makes sense when it does contain a time zone specification.

Your code works on my machine. Thanks!

Evidently CST is ambiguous because there is a China Time Zone.
zone_map{} appears to be necessary.