Diarmuid does understand, or at least as much as he can. He's not sure when his birthday is here because of how different the calender is, but he's sure it has passed by now and the idea of his father not being here to celebrate with him makes him sad. Kit is probably feeling the same way, only worse.

"This is the first time you have been away from him when it happened and it's not the same. He should be here..."