Well Clinton did preside over the dot com boom, leaving Bush to clean that up. He was the first democrat to be president since Carter. Not sure your thesis is very strong there as it depends upon an arbitrary duration, a nebulous criteria and an implication that democrat admins have behaved differently (based on same nebulous criteria). In reality, policies of presidents can be very loosely coupled to outcomes during their time in office. Clinton’s policies, for instance, are not insignificantly indicted by the mortgage crisis as well, but just because the chickens didn’t come home to roost before he left office doesn’t prevent us from seeing that. There’s a long tail to policy decisions for better and worse.