Not sure on frost, the data BTV keeps is "freeze" climatology... but even the notorious cold pockets like Lake Placid and Saranac Lake are decently into October for record late.
Like the mean first freeze at Saranac Lake is September 10th (so already passed) but record early is August 20 and late is October 13. So there's a pretty wide range of possibilities there it seems.
Island Pond, VT is similar first/last wide range.... late August to mid-October with a mean in mid-September.
Again that's 32F freeze climo, not necessarily frost which can happen in the mid-30s.
I would assume those cold spots could be comparable to HIE, so likely late record 32F maybe in that October 5-15 range?