my $dt = DateTime->new(
    year      => 2021,
    month     => 4,
    day       => 5,
    hour      => 2,
    time_zone => 'Africa/Nairobi',
);
# or, create a DateTime object from an ISO timestring
my $isostr = '2021-10-11T11:13:57';
my $dt = DateTime::Format::ISO8601->parse_datetime($isostr);
# add a second to what $dt currently holds
$dt->add(seconds => 1);