The clocks were striking thirteen

The very first line of nineteen eighty four is "It was a bright cold day in April, and the clocks were striking thirteen." The book begins on this absurd note. Clocks do not strike thirteen anymore than ,well, two plus two equals five. True there is military time but I don't think that was what Orwell was trying to convey. His intention was something more poetic. He is suggesting that something is wrong, something absurd about the world we are entering.

I don't think "the clock striking thirteen" is common enough to be considered an idiom.  In addition to "Nineteen eighty four" shows up in only a few other places notably in A.P. Herbert's "Uncommon Law", a collection of fictional legal cases which were original published in the British satirical magazine “Punch”. The fictious case in question is “Rex vs. Haddock” subtitled “Is it a Free Country?” the full text of which can be found here. https://www.scribd.com/document/124640692/Rex-v-Haddock

The case involves a man being arrested after jumping off the Hammersmith Bridge “for fun”. He was charged with a number of offenses each of which he attempts to explain or supply a defense... 

But in addition to these particular answers, all of which in my judgment have substance, the appellant made the general answer that this was a free country and a man can do what he likes if he does nobody any harm. And with that observation the appellant's case takes on at once an entirely new aspect. If I may use an expression which I have used many times before in this Court, it is like the thirteenth stroke of a crazy clock, which not only is itself discredited but casts a shade of doubt over all previous assertions. For it would be idle to deny that a man capable of that remark would be capable of the grossest forms of licence and disorder. It cannot be too clearly understood that this is not a free country, and it will be an evil day for the legal profession when it is. The citizens of London must realize that there is almost nothing they are allowed to do. Prima facie all actions are illegal, if not by Act of Parliament, by Order in Council; and if not by Order in Council, by Departmental or Police Regulations,or By-laws. They may not eat where they like, drink where they like, walk where they like, drive where they like, sing where they like, or sleep where they like. And least of all may they do unusual actions 'for fun'. People must not do thing sfor fun. We are not here for fun. There is no reference to fun in any Act of Parliament. If anything is said in this Court to encourage a belief that Englishmen are entitled to jump off bridges for their own amusement the next thing to go will be the Constitution. For these reasons, therefore, I have come to the conclusion that this appeal must fail. It is not for me to say what offence the appellant has committed, but I am satisfied that he has committed some offence,for which he has been most properly punished.

Now whether Orwell was making a direct reference to this piece is not certain. “Uncommon Law” was published in 1935 so it is indeed possible that Orwell read it.  They are both British. It is undeniable that the reference would make sense. "Rex vs. Haddock" is much more light hearted than Nineteen Eighty Four but they clearly share the same theme. Despite it's overwhelming darkness Nineteen Eighty Four is a satire. It would not be surprising if the one satire borrowed a line from the other.