I started a program and wrote the output of ps -p PID -o lstart= to a file, like so:$ long_running_command &[1] 4562$ ps -p $! -o lstart= > start_time$ cat start_timeWed Apr  6 06:16:31 2016But when I re-run ps later I get a slightly different result:$ ps -p 4562 -o lstart=Wed Apr  6 06:16:53 2016I was under the impression that lstart= would print the start time of a given process. Why am I getting a different start time when I re-call ps?
Output of ps lstart changed
linux;ps