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 <span>Far From the Madding Crowd,
"</span><span>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."</span>