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The output of your command "$ last |grep system | head -5" is:

reboot   system boot  6.8.0-55-generic Wed Apr 30 09:49   still running
reboot   system boot  6.8.0-55-generic Tue Apr 22 19:29   still running
reboot   system boot  6.8.0-55-generic Sat Apr 12 10:17   still running
reboot   system boot  6.8.0-55-generic Sat Apr 12 09:34   still running
reboot   system boot  6.8.0-55-generic Fri Apr 11 00:13   still running