$cowtime->timestamp->set_formatter($formatter);

##</code><code>##

use 5.12.0;
package CowTime {
    use Moo;
    use Types::DateTime -all;
    has timestamp =>
        is => "ro",
        isa => DateTimeUTC->plus_coercions( Format['ISO8601'] ),
        coerce => 1,
        trigger => sub {
          my ($self, $value) = @_;
          state $formatter = DateTime::Format::MyFormatter->new;
          $value->set_formatter($formatter);
        };
    1;
};