Not asking about any particular concept of well being or happiness. The question may seem odd: if I am happy today now then I will have been happy for the day. But I suspect it at least may be more complex than that.
Does eudemonia depend on tense or does change only show us that our happiness can be unreal?
Your question uses both "eudaimoneia" and "happiness" as if they were interchangeable. But "happiness" has two meanings. One might be called "life satisfaction" and the other "affect-based", though neither of these terms is quite satisfactory. "Eudaimoneia" points firmly in direction of life satisfaction in an objective rather than a subjective sense; it is often translated as "being fortunate".
There are two issues here, but they interlock.
One is the issue between a concept of the good life that accepts that the good life depends on circumstances outside the agent's control and one that doesn't. Aristotle is firmly in the former camp. This is the issue of moral luck.
The other issue is whether eudaimoneia is a permanent, or at least life-long, state or whether it can be temporary. This seems to be the question you are asking.
Martha Nussbaum in her book "The Fragility of Goodness" argues that it can be temporary. Her argument is developed in the context of Ancient Greek philosophy and tragedy (which makes the book unusual in philosophical literature). She appeals to the tragedies, arguing that they show that people who are deeply committed to justice are vulnerable to external circumstances. She considers how far reason enables eudaimoneia to be independent of the chances of life and contrasts this view with Plato's idea that goodness is not affected by the circumstances that seem to threaten it.
The key issue, it seems to me, is whether eudaimoneia can be preserved in the face of an insoluble moral dilemma or not. If it can, it would seem that behaving well in such circumstances is enough. (But it may be impossible to pin down what "behaving well" is in such circumstances.) If it cannot, then eudaimoneia is not a question of what one deserves. This seems morally repugnant.
There is another issue relevant to this - whether eudaimoneia may be a question of balance over the whole of a life.
There is a review of "The Fragility of Goodness" at The Fragility of Goodness. Luck and Ethics in Greek Tragedy and Philosophy. (Revised edition) – Bryn Mawr Classical Review
There is a more general discussion of these issues at Happiness (Stanford Encyclopedia of Philosophy)
