Global clocks aren't required, as batch intervals on different exchanges don't need to be synchronized. Although the batch interval is a window of cross-exchange risk, the uniform-price clearing means it is still more efficient (in "volume-space"). Continuous auctions have really short risk windows (more efficient in "time-space"), but the discriminatory pay-as-bid price clearing means that all that risk is shouldered by everyone but the fastest traders (who are thus able to extract arbitrage rents from the slower traders).