To me, the _real_ thing that matters isn't quite date or budget, but something that somehow acts as an umbrella to both of them: the promise. When you promise to deliver something by a day, or within a budget, it's very clear whether you met your promise or didn't. However, when it comes to functionalities, there is more of a grey area: you can start to argue that something _mostly_ works, that some bugs are always inherent, or that this functionality actually is not really needed because the problem can be fixed in an operational way, or that the requirements have changed, or that it was just a nice-to-have... but money/time don't have this grey areas.