Comment by hi-v-rocknroll

There is no PID 0, it's a ABI convention just as using a negative PID is a convention to use the PGID instead like kill -9 -1. PIDs start at 1. The Linux kernel allocates PIDs to kernel threads that are "processes" without a separate address space running in a privileged mode with separate stacks, and generally ignore kill() signals.

Command to get when a Linux box was started as opposed to just running uptime -s:

    ps -p 2 -o lstart=