my $date_constraint = Moose::Util::TypeConstraints::find_type_constraint('MyApp::DateOnly');

around BUILDARGS => sub {
    my $orig  = shift;
    my $class = shift;
    my %args  = ( @_ == 1 ? %{ $_[0] } : @_ );

    # Maybe[] doesn't coerce
    $args{$foo} = $date_constraint->coerce($args{foo})
        if defined($args{$foo});

    return $class->$orig(%args);
};

has foo => (
    reader  => 'get_foo',
    writer  => '_set_foo',
    isa     => 'Maybe[MyApp::DateOnly]',
    coerce  => 1,
    handles => {

        set_foo => sub {
            my ($self, $arg) = @_;
            # Maybe[] doesn't coerce
            $arg = $date_constraint->coerce($arg)
                if defined($arg);
            $self->_set_foo($arg);
        },

    },
);