Linux uptime in seconds

uptime

 17:35:49 up 1 day,  7:03,  3 users,  load average: 0.00, 0.00, 0.00

# Shows how long the system has been on since it was last restarted.

cat /proc/uptime

111848.31 892814.41

The first value represents the total number of seconds the system has been up.

The second value is the sum of how much time each core has spent idle, in seconds.