This is probably gonna be one of my rare short answers:
- 年内: ‘within the year’
- 年中: ‘throughout the year’
You’ll find that both of these line up with other uses of 内 and 中, including in similar compounds.
If that’s not enough because you haven’t been exposed to much Japanese yet, and you need an extra boost, then try this: I said ‘throughout’, right? Doesn’t the central stroke in 中 go through it? No such thing for 内, but I mean, you could say that the 人 is within the 冂. I hope that helps.