What sort of PT schedule are you looking for -- i.e. full days but only 2-3 times per week, or mornings only but 5 days a week, etc.? I ask because many preschools would fit the bill for mornings 5 days a week but not necessarily for full days fewer times per week.
Most preschools start at age 2 and are on a school schedule (i.e. 2 by Sept 1 or Oct 1), but some have rolling admissions. I'm guessing you're looking for a place for a younger child with the option to continue as he/she gets older, but it's an option to do preschool as your kid gets a little older to fill the place of PT daycare. I also have seen some preschools that do offer full-day care but only 2 or 3 days per week.