In the new history, Longstreet has been expecting Herdeg to arrive. He did not know what year it would be, but he was certain that it would be on May 3rd. That is passing peculiar.
The doctor's reasoning goes something like this: on May 3rd, 1943, he opened his end of a time portal, and Dave Herdeg vanished into it. On May 5th, 1943, a nuclear armed stealth bomber, clearly from the future, drops its payload on Washington. It must have traveled through the portal Longstreet opened, and therefore Dave must have exited at the other end.
We have already seen that this is the wrong David; what is peculiar, though, is why Longstreet thought the other end must be May 3rd. What opens the other end of the wormhole is not the date but the fact that there is a second experiment generating enough power for the two to connect through hyperspace. Or is there another factor?
The writers might be attempting to avoid the perpetual question, why does the time traveler wind up on earth? After all, this question posits, the earth is constantly moving, spinning on its axis and orbiting the sun. It would take a year for the planet to be in the same place it was. So then, maybe we only get these connections if we conduct these experiments at a moment when the earth is in the same place as it was. That way there is only one point in space, but two points in time.
It is a nice theory, but flawed: the sun is also moving, and the orbit of the earth is shifting slightly such that its distance from the sun is slightly different. The same date would be irrelevant. Further, Longstreet would know this. He would presumably take into account the shift in the position of the galaxy in calculating when the earth would be in the "same place". It is not my field; I am not certain whether the earth is ever in the "same place"--some in relativity would say that there is no such location, and it certainly would be difficult beyond measure to identify, with the earth moving around the sun, the sun moving around the galaxy, and the galaxy itself moving relative to other galaxies. So scientifically it proves to be nonsense.
Yet even were that not so, there is no reason for Longstreet to think the location of the planet matters. He knows that the bomber arrived on the day he opened one end of the vortex; he can reasonably conclude that Herdeg would arrive on the day that someone opens the other end. That could be any day of any year. Further, he knows that German Mailer is performing experiments to attempt to travel to the past, but he does not and cannot know whether he has as yet created a portal.
On top of that, although in the first film we do not know exactly when Dave and Jim arrive in 1984, we do know that when they ask Allison the date she says, "the 17th". Even without knowing the month, we can see that the 17th is at least half a month away from the 3rd, whether it is May 17th, (17 minus 3 equals) 14 days later, or April 17th, (30 minus 17 equals 13, plus 3 equals) 16 days earlier. They did not then arrive on the 3rd, and there is no reason to expect that they would.
Of course, Longstreet does not know that, because in this history that never happened; we know it, though, and it makes our acceptance of these events more difficult.
That raises another question, in terms of an inconsistency with the nature of time travel in the first film.