Run coercions inside a Type::Tiny custom constraint?

I have a custom DateTime type that has a coercion defined on it from string to DateTime, like so:

package Library;
use Type::Library -base, -declare => qw(DateTime);
use DateTime::Format::ISO8601;

class_type DateTime, { class => 'DateTime' };

coerce DateTime, from Str, via { DateTime::Format::ISO8601->parse_datetime($_) };

I'd like to use that DateTime type in a Dict, like so:

package MyObj;
use Moo;
$constraint = declare MyType, as Dict[ name => Str, date => DateTime ];

has 'whatsis' => ( is => 'ro', isa => $constraint );

And then call it as:

use MyObj;
my $obj = MyObj->new( whatsis => { name => 'Something', date => '2016-01-01' } );

I've tried adding coerce => 1 to the declaration of whatsis, but that did no good.

How do I create a custom type that inherits from Dict and runs the type coercions defined on the member types?