NYC doesn't have the "almost all apartments operate on a Sept 1 - Sept 1 schedule", so if that's true in Boston it makes sense for there to be a fairly different ecosystem there.

I'm used to the NYC rental market, particularly Brooklyn, and, aside from lining up apartment-mates, the rule is that you look for a new apartment right before you're reading to move.

NYC has a very different apartment listing culture than Boston, yup!

Where are you getting your listings and how can you tell when the lease is intended or expected to start from the listing?

I'm scraping Padmapper. The availability date is usually plain text in the listing, unfortunately, and is also not something I have in my archived date (just location, price, and number of bedrooms).

But if you go on Padmapper, Craigslist, etc in April and look at listings, you'll mostly see 9/1 start dates.