We truly don't know how long some vaccinations last. Some are known to last only a bit more than one year. Some are generally accepted to last at least three years. Some researchers vaccinate their dogs or cats as puppies or kittens, a year later, then never again… although there isn't enough verification of protection for most veterinary practices to feel confident with this little protection.
Why don't we know more about duration of immunity (DOI)? Who is going to pay for these very expensive studies? The vaccine manufacturers aren't going to make an attempt to prove you don't need their product. The universities have more pressing medical studies when budgeting their research money. The individual veterinary practices? Your veterinarian may discuss longer intervals between vaccinations, but that is based on gut feel from experience. Even the big corporate practice groups aren't going to risk not vaccinating pets in an attempt to acquire this data. We do have generally accepted intervals published by the American Academy of Feline Practitioners and the American Animal Hospital Association, based on what the veterinary community does know.
As for the vaccine being due “tomorrow”, don't sweat that. Science doesn't (and probably can’t) design a vaccine that protects for exactly 365 days. I really hate that veterinary software packages all assign a specific date to vaccination expirations. We advise our clients that any time within a month of that is sufficient.