There are several ways to find out the last system reboot time,
The first method is to use last command.
This command actually shows system uptime for the last few days. Originally the last command is designed to show login history of a particular user. In Linux, a special "pseudo user" named reboot automatically logs in to the system right after the system has rebooted. Thus by checking the login history of reboot user, you can check the last reboot time.
Another command to check the time of last system boot is to use who command with '-b' option.
You can also use uptime command to deduce last reboot time. The uptime command shows you the current time, as well as how long the system has been running. From this information, you can calculate the time when the system was last booted.
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