Re: City Parks established dates?
Assuming the park is commensurate with the surrounding neighborhood's home building dates (like ..... if the park was built at the same time of the growth of that part of the city, ... home-wise), then all you need to do, is determine the age of the surrounding homes. I've done this with the aid of zillow and realtor.com. Simply find homes for sale in the area, click on "detail", and it usually shows you when the home was built.
But this assumes the park age does not predate the growth of the city to this particular area. This isn't always the case, because perhaps a park existed there in prior times, and the city grew up around it, for example. But at a minimum, this tactic would give you an "at least" date to go by. And for the typical square block-ish parks, where the tree types/sizes also do not appear to predate the homes (ie.: not magnificant oaks that look like an older scenic grove type destination), then it's safe to assume that the park plan was laid out with the city planning upon growth to that part of town.
Trying to find the oldest park in my area! 
