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

reboot   system boot  6.8.0-60-generic Mon Jun 16 20:54   still running
reboot   system boot  6.8.0-60-generic Mon Jun 16 20:44   still running
reboot   system boot  6.8.0-60-generic Mon Jun 16 18:49 - 20:02  (01:13)
reboot   system boot  6.8.0-60-generic Mon Jun 16 17:20 - 20:02  (02:42)
reboot   system boot  6.8.0-60-generic Mon Jun 16 14:07 - 17:20  (03:13)