Plot hole: Dan returns from the future with a vial of toxins with him that can kill all the white spikes. He tells somebody to start mass-producing it but this information is not spread across governments or even made public, but it is the one thing they have that can prevent humanity's extinction and making people aware of that will stop the rioting.
Plot hole: So humanity in the future managed to solve time-travel, but is unable to develop armor-piercing, exploding ammunition? And it uses old, untrained civilians as soldiers to fire M16-16 and WW2 heavy machine gun ammunition at fast moving targets, instead of developing rapid-fire computer-targeted guns? The movie shows 0.5" ammunition (from said WW2-era machine gun) CAN take out the Aliens, so? (Not to mention flame-throwers, shown as effective but sparsely used, no tanks, etc).
Factual error: The aircraft launched to destroy the city of Miami are F-22 air superiority fighters, which are entirely incapable of saturation or carpet bombing. Even the way they carry out the operation in the movie demonstrates this. They blow up a few streets and that's it. What was the point?