Change Stream Timestamp (clusterTime) to Date


I tried to convert “clusterTime” from the change stream (oplog) output like that:
new Date(chunk.clusterTime.high *1000)
the result is 4 minutes! later than what I have expected.

this object is typed as Timestamp and has isLong = true property

how should I convert it to JS Date?
this is my only way to see if the change stream is reacting fast enough or not…