__PACKAGE__->has_a(
    startdate => 'Date::Class',
    inflate => sub { Date::Class->new(shift) },
    deflate => 'some_method_name'
);

##</code><code>##

SomeTab->create({startdate => '07/08/03'});

##</code><code>##

SomeTab->create({startdate => Date::Class->new('07/08/03')});