In the book 1984, what does the opening sentence suggest about the book?

The thirteenth stroke here doesn't refer to military time but to an old saying. References to a thirteenth stroke of the clock indicate that some event or discovery calls into question everything previously believed. Put another way, the thirteenth stroke of the clock calls into question not only the credibility of itself but of the previous twelve.

But notice in this opening line that it isn't just one clock malfunctioning, but the clocks. Presumably all of them. In this world, the clocks striking thirteen is not an aberration, but a normal way of life.

In this way, Orwell subtly alerts the reader that statements of truth in this fictional society should be called into question.

pulling in the reader is important