From the DateTime::Format::Strptime SYNOPSIS: on_error

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

my $t2 = 'Sep 16 21:00:00';

my $Strp2 = DateTime::Format::Strptime->new( 
    pattern     => '%B %d %H:%M:%S',
    locale      => 'en_US',
    on_error    => 'croak',
);
my $dt2 = $Strp2->parse_datetime($t2);
print "'$dt2'\n";

__END__

There is no use providing a month name (Sep) without providing a year.
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