20 Jun
                
                    2016
                
            
            
                20 Jun
                
                '16
                
            
            
            
        
    
                11:42 a.m.
            
        Actually they didn't reboot but uptime graphs restarted. Can be a kind of counter overflow ?
Uptime is presented from the returned value in sysUptime.
observium~$ snmpwalk -v2c -c public device sysUptime DISMAN-EVENT-MIB::sysUpTimeInstance = Timeticks: (106822559) 12 days, 8:43:45.59
That would be the first place I would look. It may just be that the SNMP daemon on the device has restarted (which is what that value is actually reporting the uptime of).
Cheers,
Michael