There are four ready ways to figure out when your computer was booted (or rebooted):
Task Manager Shows Up Time:
Command Prompt To Show UpTime:
systeminfo | find /i “Boot Time”
System Boot Time: 20/03/2015, 12:03:04 PM
PowerShell 3 Command To Show Boot:
Get-CimInstance -ClassName win32_operatingsystem | select csname, lastbootuptime
For more information see THIS TechNet article.
WMI Command To Show Boot Time:
wmic os get lastBootUpTime
Note that this is in Milliseconds.
For more information see THIS article.