This is an undocumented bug (at least as far as I can tell), but basically, it occurs in situations where the execution time is so small it causes an integer underflow, and instead returns an absurdly large value as a result. I do find it curious that you got such a "small" number, as this bug usually manifests as a duration of 2,147,483,647 milliseconds, which is equal to the binary value -1 as a signed 32 bit value.
Until/unless we get an official answer, we simply have no way of knowing the "why", all we know is that it can and does occur with very small execution times. I've ever specifically explored the limits of this, but it's likely only a problem if the time is less than a few milliseconds. By the way, I managed to get a value of 484,521,164 when I replicated your bug using nearly identical code. It seems that the bug is more random than it used to be.
This effect is completely harmless, aside from showing really large duration times in the debug logs. It doesn't mean that your code stalled for 180 hours, that you've invented time travel, or anything else. It's a simple integer underflow that you can safely ignore. I've known about this issue for years, I've just ignored it, since it doesn't have any real material effect, unless you're trying to do something with it, since logging is not always enabled, you wouldn't know or care about this under most normal circumstances.