The Resolute Urgency of Now

Buffy has fought for the side of the not-so-righteous for nearly 5 years. She is no closer to knowing her future; what choices she has or what fate has in store. In a life with a limited future, 'now' has an urgency to it. What if every slayer came with an in-built expiry date, to prevent them becoming too old and less effective, or from reneging on their duty?