Pardon, it's billion indeed if you count seconds. I wrote millions for mistake, but thinking about it once you have 64 bits, milliseconds are nice to have...
But once you have milliseconds, microseconds are nice to have...
The linux kernel (and many other applications) solve this with a tuple of 64-bit ints (seconds, nanoseconds) where 0 <= nanoseconds <= 999999999. Compare this to simply 64 bits of nanoseconds, which would run out in roughly 2554 CE.
Other systems still (perhaps most commonly) are using double floats for seconds. Under that scheme, nanoseconds were only representable until Feb 17th 1970. The last representable microsecond will be some time in 2106, and the last representable second won't be for another 150 million years or so.
Personally, I'm happy with the precision afforded by floats. Timing uncertainly (outside niche applications) is generally much larger than a single nanosecond, and even microseconds are a bit suspect.
