And how do we know this? The timeline of the events of Skyrim that is. It hasn't been revealed exactly when it will be taking place.
The few quotes from the trailer may give a hint about the time:
"They're already here."
"The Elder Scrolls told of their return."Found no clue yet where exactly in the ES is a hint for their return.
"Their defeat was merely delay"Their (Dragons) population in Akavir was killed by the Tsaesci, whereas the dragons in Vvardenfell were overrun by the invading cliff racers. It was rumored that they moved closer to Cyrodiil, and it may be for this reason that dragons are sacred to the Empire. The legends tell of their aid during wars in exchange for shelter.
"Til the time after Oblivion opened,"
"When the sons of Skyrim would spill their own blood."After quarrels from the rivalries of the Ysgramor clan however, Skyrim weakened and separated into multiple states during the War of Succession.
1E 369 - 1E 420
When King Borgas of Skyrim was killed by the "Wild Hunt" in 1E 369 the Moot, arbiters of the succession, failed to choose the obvious canditate, Jarl Hanse of Winterhold, and sparked a civil war.
The war was disastrous for Skyrim: it lost the provinces of High Rock, Morrowind and Cyrodiil and cities such as Windhelm were sacked. In 1E 420, the Pact of Chieftains brought an end to the conflict.
The Oblivion crisis took place in the 3rd era (3E 433), but the Sons of Skyrim battled each other during the 1st era (1E 369 - 1E 420). I'm sure the planes of Oblivion already existed during that era since Jauffre told us:
"Close shut the jaws of Oblivion"...?"
"His meaning is unclear to me as well. The Emperor seemed to perceive some threat from the demonic world of Oblivion. The Prince of Destruction, Mehrunes Dagon, is one of the lords of Oblivion. But
the mortal world is protected from the daedra of Oblivion by magical barriers."
Edit: The Book "On Oblivion" quotes: "It is improper, however customary, to refer to the denizens of the dimension of Oblivion as “demons.” This practice probably dates to the Alessian Doctrines of the First Era prophet Marukh -- which, rather amusingly, forbade “trafficke with daimons” and then neglected to explain what daimons were."
Could it be possible that the Oblivion reference during the trailer doesn't mean the Oblivion crisis from TES IV ? Maybe "Oblivion opened" happened already before in the 1st era and the Dragons helped closing back then ?