I'm going on student exchange to Montreal at the beginning of next year and to get there was planning on flying to New York first and then getting a train to Montreal due to it being so much cheaper. I was then also planning on doing some travelling in the US at the end of my exchange 4 months later. However someone told me that when you apply for an ESTA, you are only allowed to visit ONCE for 90 days, so for instance, if I land in New York on Jan 1, my 90 days start then keep going until they run out, even if I leave the country on Jan 3, and then after this 90 day period I will no longer be able to enter the US again. They said because of this I should just get a direct flight to Canada and then get my ESTA at the end of my trip and only enter the US then. However I can't seem to find this information anywhere, it only says the ESTA is valid for up to two years. Have I been informed incorrectly?