Use tuptime , você obtém todas as informações de que precisa, por exemplo:
$ tuptime -e
Startup: 1 at 08:03:58 10/08/15
Uptime: 6 hours, 56 minutes and 7 seconds
Shutdown: OK at 15:00:05 10/08/15
Downtime: 17 hours, 8 minutes and 14 seconds
Startup: 2 at 08:08:20 11/08/15
Uptime: 6 hours, 51 minutes and 38 seconds
Shutdown: OK at 14:59:58 11/08/15
Downtime: 17 hours, 7 minutes and 46 seconds
Startup: 3 at 08:07:45 12/08/15
Uptime: 6 hours, 50 minutes and 47 seconds
Shutdown: OK at 14:58:32 12/08/15
Downtime: 17 hours, 5 minutes and 18 seconds
Startup: 4 at 08:03:51 13/08/15
Uptime: 6 hours, 55 minutes and 12 seconds
Shutdown: OK at 14:59:03 13/08/15
Downtime: 17 hours, 14 minutes and 20 seconds
Startup: 5 at 08:13:24 14/08/15
Uptime: 1 hours, 28 minutes and 14 seconds
System startups: 5 since 08:03:58 10/08/15
System shutdowns: 4 ok - 0 bad
Average uptime: 5 hours, 48 minutes and 24 seconds
Average downtime: 13 hours, 43 minutes and 7 seconds
Current uptime: 1 hours, 28 minutes and 14 seconds since 08:13:24 14/08/15
Uptime rate: 29.74 %
Downtime rate: 70.26 %
System uptime: 1 days, 5 hours, 2 minutes and 1 seconds
System downtime: 2 days, 20 hours, 35 minutes and 39 seconds
System life: 4 days, 1 hours, 37 minutes and 40 seconds