A number of people mentioned The Doomsday Book. I really didn't like that book. A lot of people have enjoyed it as a fantasy, because it's really bad SF.
The time machine was merely a contrivance for her to write a book about the plague, and that is ultimately its downfall (at least for me). It destroyed my suspension of disbelief. No, I take that back. It didn't just destroy it, it jumped out of the book, ran around my house knocking over lamps, searching for every last scrap of my suspension of disbelief to utterly annihilate.
A time machine would harness more power than several thousand stars and would be the most potent force of potentiality on the planet. And where is it? Handled by a bunch of academic boobs in a public university.
A society that can build a time machine could equip their time travelers against all potential harm and contingencies. A time traveler would be better equipped and better trained than any current soldier/astronaut. And who do they send back? A grad student.
A large portion of the plot revolves around the fact that at this time, far into the future of human society where they are building time machines for universities, the characters keep missing each other because they have neither cellphones nor answering machines. Bleh.