This Year's Model of Humanity

I think the point that principia_coh is trying to make is that, unless Martha has an eidetic memory, spending one or two subjective years away from her studies means that she's lost quite a bit of knowledge, through lack of use and lack of additional study. It's not that she's missing objective time, but that when she comes back, she's not going to remember where she left off, because she hasn't been doing this for two years now.