The environment in which the protagonist, Winston Smith, survives is carefully drawn in Part 1. People are constantly under surveillance, (or made to think they are), with posters of Big Brother everywhere, and telescreens that are never turned off.
To maintain control, a number methods are used. Interestingly, these methods have become more prevalent since 2017.
The Government lies to the people, facts are altered. It is a society where things deteriorate but the official propaganda tells them things are better.
Winston Smith works at the Ministry of Truth. His job is to spread lies and destroy evidence. Hence if last week a ration was X and this week it is changed to 1/2 X he is to report it as being increased to 1/2X.
He knows, for a fact, he is altering the past to match the present.