OP, from the nanny side it's pretty tough to imagine someone looking for a position now that doesn't start until the winter (at the earliest). The people who will be available/looking for work now won't wait until then, and the people who are available then aren't right now (and probably the majority don't know that they'll be in a job search). I'm sure there are exceptions, but if you search/hire now for a position that doesn't start for possibly almost a year the odds of you getting the best pool of candidates, and the best fit for your family, aren't great. Also, the odds of something falling through between now and then are pretty good - there's every chance you'd have to do the whole search over again anyway.
I really think it's too soon to being searching for a nanny. Daycare, as others have said, is something else - that you do need to be exploring, getting on lists for, etc...