It is less complicated than it looks, once the terms are clear.
Most people meet this once and never think about it again, which is exactly why it is worth reading up on.
The exceptions
Nobody is required to explain this to you, which is half the problem. Most of the confusion comes from two terms that sound alike and mean different things.
What it costs
The exceptions matter more than the rule, and there are three of them. Nobody is required to explain this to you, which is half the problem.
What to check first
The practical effect is easier to see in a single example than in a summary. Most of the confusion comes from two terms that sound alike and mean different things.
The short answer: check the date, then check the figure, then decide.