Re: I love WSL, since it made scripting on Windows usable after 30 years :)
Try that in PowerShell.
Ok then:
Get-CimInstance -ClassName win32_operatingsystem | select lastbootuptime
(Note how instead of having to manipulate strings using multiple programs, we just grab the one field we're interested in, which is returned as a DateTime so we don't have to reformat it if we wanted to use it in a script.)