1984 by George Orwell! :D
The opening is unclear + ambiguous but some say it opens up like this for one of three reasons:
1.) Military time
2.) Because clocks don't strike 13, it shows that there is something wrong with the clock and it is untrustworthy. Just as in the story, 2 + 2 = 5. The government tells lies and has lost respect for the truth.
3.) Orwell is referencing Thomas Hardy from his novel Far From the Madding Crowd,
"This supreme instance of Troy's goodness fell upon Gabriel ears like the thirteenth stroke of crazy clock. It was not only received with utter incredulity as regarded itself, but threw a doubt on all the assurances that had preceded it."