Have you ever noticed how horrible Rick is at math? Great at character building, but bad at math.

Example 1. In The Lost Hero, it is December. In the short story, "Leo Valdez and the Quest for Buford" it is stated that two months have passed since Leo, Piper, and Jason came to camp. It is Christmas Eve. But if it was December in The Lost Hero, how can it be Christmas Eve two months later? Did Camp Half Blood get stuck in a time loop?

Example 2. Percy and Annabeth start dating in August. Percy disappears in December. That's four months of dating before Percy's disappearance. Annabeth actually says so. But when Percy gets his memory back, he says they dated for two months before his disappearance. This can be blamed on the fact that Percy's mind is still muddled.

Example 3. Nico is stated to be 12 in The Last Olympian. Annabeth is 16. Percy turns 16. The blood of Olympus takes place almost a year later. Annabeth has just turned 17. Percy is almost 17. So Nico should be 13. But he's 14. He skipped a whole year. And Hazel, in order to still be younger than him, would have to be 12. Then, in Trials of Apollo, when he should be 14, he's 15. In Heroes of Olympus, Will is a little older than Nico. But then in Trials of Apollo, they're the same age. 

But we love you anyway Rick. Other aspects of your work make up for your lack of mathematical skill. So we don't care. I just noticed these things recently. J.K. Rowling has similar problems.