I can’t take credit for finding this myself. Gilad Gurantz forwarded it to me. From the Quartz Event Services documentation:

CGEventTimestamp

Defines the elapsed time in nanoseconds since startup that a Quartz event occurred. typedef uint64_t CGEventTimestamp; Discussion An event timestamp is a big, unsigned, 64-bit number. That's big, really big. You just won't believe how vastly, hugely, mind-bogglingly big it is. You may think your application has been running for a long time, but that's just peanuts to an event timestamp.

So if my math is right, “a long time” translates into over 2850 years. And it’s counted since the last reboot. I think we’ll avoid the Y2K problem on this one.