I don't know why you need a great clock either, if you have stable, symmetric network paths from a central location to all your servers colocated at exchanges, you can predict the delay between sending from the server and getting to the exchange, you can split your order and send it to the various exchanges with appropriate delays and know that everything will arrive at the same time. If you're wrong, it's going to still be close enough that nobody will be able to see it on one exchange and react to it on another before your order gets there.
This was the first thing I thought of when hearing the flash boys story on the radio: The banker was complaining he couldn't capture the whole book across exchanges because resting orders were cancelled before his order got there -- he just needs to get his orders to arrive close enough in time (although expect a bigger tick, probably)
I've evolved to keep this rule in mind when I hear almost any story/portrayal/claim/news anymore. Especially anything that feels sensational or too-good-to-be-true-at-first.
For example, people often lie about sex or money. National governments often lie about, well, pretty much anything that suits their best interests. When I say "lie" I don't mean completely wrong or totally false, merely, spun a certain way, sometimes a careful omission of critical modifiers, the use of weasel words, etc.
Network delay variance over distances is much greater than the timescales at which HFTs function. That's why HFT equipment is always so close to the exchange itself.
But it's not significant in the domain this thread is focused on. In order to lose signal to an rival in this scenario, the delay between your Order A hitting Exchange A and your Order B hitting Exchange B would need to be sufficient to give the rival time to learn the signal, and then to act on it. At a minimum, that is half the cost of the round-trip time between the Exchange A colo and the Exchange B colo. So long as you're not introducing such substantial delays in your order dispatch, you're fine.
This was the first thing I thought of when hearing the flash boys story on the radio: The banker was complaining he couldn't capture the whole book across exchanges because resting orders were cancelled before his order got there -- he just needs to get his orders to arrive close enough in time (although expect a bigger tick, probably)