It's a slippery concept at first, but once you wrap your mind around it, it becomes clear just what a brilliant metaphor this is. Imposing a timekeeping method, a common tool of cultural imperialism, becomes a weapon of plain old ordinary imperialism. The Hexarchate propagates itself by literally winning over hearts and minds, forcing people to live according to its calendar (or risk being suppressed by one of the many arms of its doctrine-enforcing police force), which gives it the power to continue oppressing them. And, in order for any rebellion against the empire to succeed, it has to impose its own calendar, which is to say its own way of seeing the world, on a sufficiently large population.
I actually ended up liking Revenant Gun rather less than the two previous volumes in the series, Ninefox Gambit and Raven Stratagem. Its focus ended up being a lot less on the area I was interested in, the complicated problem of building a better society in a setting where calendrical weapons and technology are the dominant paradigm, and more on the character of Shuos Jedao and his quest for redemption, or at least a level of monstrousness he can live with. I found Jedao rather problematic (and honestly, not that interesting) in the first two books, and the increased emphasis on him was frankly rather tedious. (Also, this is maybe not the best time to be telling stories about tortured, justified killers; we keep seeing real-world examples of how society bends over backwards to make excuses and try to read goodness into utterly depraved people, and it should be obvious that the character type of Jedao comes from the same place.) If I were recommending this series to people, I think I would tell them that the first two books work perfectly well as a duology about rebellion within the Hexarchate, and to only read Revenant Gun if you're particularly invested in the character of Jedao.