It's related to MySQL's timestamp's time zone being unknown ('floating').

use strict;
use warnings;

use DateTime                qw( );
use DateTime::Format::MySQL qw( );

my $now = DateTime->new(
    year      => 2009,
    month     => 10,
    day       => 5,
    hour      => 14,
    minute    => 48,
    second    => 9,
    time_zone => 'America/New_York',
);

my $then = DateTime::Format::MySQL->parse_datetime(
   '2009-10-06 18:00:00',
);

for (1..2) {
    my $diff = $then->delta_ms($now);
    print( $diff->in_units('minutes'), "\n" );

    $then->set_time_zone('America/New_York');
}
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I don't know why it doesn't give an error.