Thirty-Three Days - Stephen King B

Jenny is a lonely university lecturer who's consciousness has traveled back in time to her younger body to try to save the future of the world. A young microbiologist is going to release a genetically modified wheat that will mutate and ultimately destroy all plant life, leaving nothing but barren windswept dust bowls.
In the past, Jenny finds a love that has been missing from her life; the kind that comes just once in a lifetime. But Jenny can only stay in that time period for thirty-three days.
Meanwhile, in the future, fearful Jenny will fail, plans are made to send another back in time--an assassin. How can she choose between saving the man she loves or saving the future?