Machines availability on specific period of time

Oleg__D 

Maybe this?

Heartbeat
// last month 
| where TimeGenerated between ( startofmonth(now(),-1).. endofmonth(now(),-1) )
| where Computer contains "JBOX00"
// find 4th week which is week "3"
| extend maintSaturday_ = endofweek(startofmonth(now(),-1),3) -1d, maintSunday_ = endofweek(startofmonth(now(),-1),3) + 1d
// exclude 4th week from data set
| where  TimeGenerated  !between ( maintSaturday_ .. maintSunday_ )
| summarize heartbeat_per_hour=count() by bin(TimeGenerated, 1h), Computer
| extend available_per_hour=iff(heartbeat_per_hour>0, true, false)
| serialize 
| summarize total_available_hours=countif(available_per_hour==true), total_number_of_buckets = max(row_number()) by Computer, bin(TimeGenerated, 1d)
| extend availability_rate=(total_available_hours-48)*100/total_number_of_buckets
| project TimeGenerated, availability_rate
| order by availability_rate desc
| render timechart