You can use the following tools to see how long system has been running on a Linux or Unix-like system:

• uptime : Tell how long the server has been running.
• last : Show the reboot and shutdown time.
• tuptime : Report the historical and statistical running time of system, keeping it between restarts. Like uptime command but with more interesting output.

Finding out the system last reboot time and date

You can use the following commands to get the last reboot and shutdown time and date on a Linux operating system (also works on OSX/Unix-like system):

## Just show  system reboot and shutdown date and time ###
who -b
last reboot
last shutdown
## Uptime info ##
uptime
cat /proc/uptime
awk '{ print "up " $1 /60 " minutes"}' /proc/uptime 
w

Sample outputs:

Say hello to tuptime

The tuptime command line tool can report the following information on a Linux based system:

1. Count system startups
2. Register first boot time (a.k.a. installation time)
3. Count nicely and accidentally shutdowns
4. Average uptime and downtime
5. Current uptime
6. Uptime and downtime rate since first boot time
7. Accumulated system uptime, downtime and total
8. Report each startup, uptime, shutdown and downtime

Installation

Type the following command to clone a git repo on a Linux operating system:
$ cd /tmp
$ git clone https://github.com/rfrail3/tuptime.git
$ ls
$ cd tuptime
$ ls
Sample outputs:

Run it

Simply type the following command:
$ sudo tuptime
Sample outputs:

$ sudo tuptime
System startups:	2   since   03:52:16 PM 08/21/2015
System shutdowns:	1 ok   -   0 bad
Average uptime: 	7 days, 16 hours, 48 minutes and 3 seconds
Average downtime: 	2 hours, 30 minutes and 5 seconds
Current uptime: 	5 minutes and 28 seconds   since   06:23:06 AM 09/06/2015
Uptime rate: 		98.66 %
Downtime rate: 		1.34 %
System uptime: 		15 days, 9 hours, 36 minutes and 7 seconds
System downtime: 	5 hours, 0 minutes and 11 seconds
System life: 		15 days, 14 hours, 36 minutes and 18 seconds

You can change date and time format as follows:
$ sudo tuptime -d '%H:%M:%S %m-%d-%Y'
Sample outputs:

System startups:	1   since   15:52:16 08-21-2015
System shutdowns:	0 ok   -   0 bad
Average uptime: 	15 days, 9 hours, 21 minutes and 19 seconds
Average downtime: 	0 seconds
Current uptime: 	15 days, 9 hours, 21 minutes and 19 seconds   since   15:52:16 08-21-2015
Uptime rate: 		100.0 %
Downtime rate: 		0.0 %
System uptime: 		15 days, 9 hours, 21 minutes and 19 seconds
System downtime: 	0 seconds
System life: 		15 days, 9 hours, 21 minutes and 19 seconds

Enumerate each startup, uptime, shutdown and downtime:
$ sudo tuptime -e
Sample outputs:

Startup:  1  at  03:52:16 PM 08/21/2015
Uptime:   15 days, 9 hours, 22 minutes and 33 seconds
 
System startups:	1   since   03:52:16 PM 08/21/2015
System shutdowns:	0 ok   -   0 bad
Average uptime: 	15 days, 9 hours, 22 minutes and 33 seconds
Average downtime: 	0 seconds
Current uptime: 	15 days, 9 hours, 22 minutes and 33 seconds   since   03:52:16 PM 08/21/2015
Uptime rate: 		100.0 %
Downtime rate: 		0.0 %
System uptime: 		15 days, 9 hours, 22 minutes and 33 seconds
System downtime: 	0 seconds
System life: 		15 days, 9 hours, 22 minutes and 33 seconds

References

• Linux last Reboot Time and Date Find Out Command
• Linux / UNIX: Getting information about reboots and shutdowns
• Linux / Unix: last Command Examples
• tuptime home page