What does the opening sentence suggest about the book?
There are two things I think are interesting about this opening sentence. One is that it is April, and this reminds me of the opening of "The Wasteland": "April is the cruelest month, breeding/Lilacs out of the dead land, mixing/Memory and desire, stirring/Dull roots with spring rain." I have no way of knowing if Orwell had this in mind, but the lines provide and excellent introduction to Winston, who has the "stirring" of rebellion in him, who lives in the "dead land."
The other thing the opening sentence reminds me of is military time. Again, I don't know if that method of referring to the afternoon is, but this clearly tells me that things are not normal, and suggest that some military or state power is running the show.
It all suggest that things are not going well ...
The novel's opening sentence is "It was a bright cold day in April, and the clocks were striking thirteen."
This sentence suggests three things. First, with the clocks striking, it suggests a known situation: a human city where clock bells chime. Second, when the clocks strike 13 rather than 1, it tells us everything is different. What's more, it isn't a single clock doing this, but all: this is a unified society, or trying to be one. Third, 13 is traditionally an unlucky number. This is the number of the last supper, and suggests someone will be betrayed and die.
I think maybe Orwell thought that this opening line was meant to cut a distinct line between the reality of his time, and the fiction that was his novel. It is hard to look back and realize just how far fetched the story was for the time, because elements of it have come true.
All the clocks struck 13, either a warning that what follows can never happen, or a a subtle hint that this is only a story, and not reality.