In Chapter Three the most significant points are made; “Who controls the past controls the future, who controls the future controls the present.”
Winston Smith is not sure of the year, or his age, for things constantly change and are hidden.
He knows that just four years ago, or thereabouts, Eurasia, the current enemy, was an ally. He remembers that Oceania, his nation was at war with Eastasia..
Yet, in his society it is;
“We are War with Eurasia, We have always been at War with Eurasia”… until it changes.
History, in Nineteen Eighty Four is fluid. It is shaped by what Big Brother determines.