Hi All,
Below node down alert solution i used for only business hours or you can say alert should work only for specific time window.
we have specific site which is working for only specific hours. so i used the below steps.
1:- Create a list of node from same site
2:- Create a custom property of node like - N_Mute.
3:- Apply the same custom property in node.
4:- Create a new node down alert and select a specific time from alert option (below image)

5:-Exclude the same node from existing node alert other wise same will be mixed
now you will get the alert for site's node only in business hours.
thanks
k