When H.G. Wells and Jane Walker arrive at the garden party in 1980, Jane tells H.G. to "Just say things like 'Madonna' and 'Wall Street' and you'll just fit in" but Madonna didn't release her first album "Madonna" until 1983. No one at the garden party would have known who he was talking about.
At the garden party in 1980, Jane tells H.G. Wells "It's the 80's. Reagan is President and greed is good" but Reagan wasn't elected President until November of that year and it is revealed that the garden party occurred in September.
A time machine can only travel back and forth through time, it cannot move to a different location unless it is physically moved in the time it is currently at so it would not be able to travel from the Vanessa's basement location to Coney Island.
When Jane and H.G. go to the garden party in 1980 she says stuff about Wall Street and "greed is good" The movie Wall Street didn't come out till 1987.
The coordinates on the piece of paper, 40.862 -73.633, are 2.5 miles from the map of the family home Vanessa was looking at, 40.892 -73.639.
When they have traveled in time to September 16, 1980, Jane tells H.G. "It's the eighties. Reagan was President...." On that date, Reagan was not yet even the President-elect, as the election was seven weeks in the future and his inauguration was over three months in the future.