Our election cycle lasts for about two months now. First, because this is true, you have a wide range of time within which people vote. Second, because in every poll some people who are deemed unlikely to vote actually end up voting, you will have people included in late polling that were not included before.
In other words, a person who pollsters deemed unlikely to vote one month before the election because he or she failed a likely voter screen will be deemed a likely voter after they have voted. They no longer fail the likely voter screen because now, unlike before, they have recently voted. This particular problem in polling advantages Democrats because they tend to vote early.
Democrats always do everything they can to get their voters out to vote before election day. Republicans always win on the actual election day itself. The question becomes whether Republicans will vote in enough numbers to offset the early Democratic vote.
This problem will produce a late surge for Democrats in polling because they are banking more of their unlikely vote than Republicans. Because Republicans deemed unlikely to vote, but who will actually vote, do not do so until election day, that vote goes uncounted in polling when the Democratic unlikely voter who actually votes will get counted.
So the long election period that we now see in most states makes polling hard. Two states where early voting is not allowed nearly as much as other states include Pennsylvania and New Hampshire. Because of this, we can probably assume the polls in those two states will more accurately reflect Trump's level of support.