What counts towards uptime?

The uptime command gets its data from /proc/uptime, which is exposed entirely by the kernel. So, we'll check out the kernel documentation to see what this actually represents.

In Documentation/filesystems/proc.txt, we see:

uptime      Wall clock since boot, combined idle time of all cpus

(there are two values in this file, hence the two descriptions)

The reference to "wall clock" is important here - it means all elapsed time, regardless of whether or not the machine's clocks are running. So, this time will keep increasing in suspended or hibernated state.

Putting it another way, the uptime value is effectively the time elapsed since the last boot.

The uptime is grossly misleading. If you use uptime -s it is more meaningful:

$ uptime -s
2019-10-31 05:30:09

Now you can see it simply shows you the last boot time. If you want to know the real uptime see this answer. Sample output:

$ suspendtime
Oct 31 05:55:19 to Oct 31 16:54:26 lasting 39,547 seconds
Oct 31 23:21:21 to Nov 01 04:29:12 lasting 18,471 seconds
Nov 01 05:51:27 to Nov 01 17:08:34 lasting 40,627 seconds
Nov 02 00:01:33 to Nov 02 10:28:46 lasting 37,633 seconds
Nov 02 18:15:59 to Nov 02 19:10:14 lasting 3,255 seconds
Nov 02 21:17:33 to Nov 03 05:31:54 lasting 33,261 seconds
Nov 03 12:06:39 to Nov 03 14:22:50 lasting 8,171 seconds
Nov 03 22:28:12 to Nov 04 04:17:13 lasting 20,941 seconds
Nov 04 05:49:40 to Nov 04 16:48:52 lasting 39,552 seconds
Nov 04 21:45:48 to Nov 05 04:19:26 lasting 23,618 seconds
Nov 05 05:52:05 to Nov 05 16:32:38 lasting 38,433 seconds
Nov 05 21:12:18 to Nov 06 04:16:50 lasting 25,472 seconds
Nov 06 05:50:45 to Nov 06 16:22:54 lasting 37,929 seconds

Linux uptime 572,689 seconds (6 Days 15 Hours 4 Minutes 49 Seconds)
13 Suspends 366,910 seconds (4 Days 5 Hours 55 Minutes 10 Seconds)
Real uptime 205,779 seconds (2 Days 9 Hours 9 Minutes 39 Seconds)