How does a VoltDB cluster determine time sync between nodes
At startup, VoltDB is going to send a round-trip message from the first node of the cluster to all joining nodes of the cluster. This message queries the local time at each node.

For each joining node, the difference between its local time and the first node's local time is computed. This includes clock skew and network latency. In fact, it's possible for clock skew and network latency to cancel each other out, but not super likely.

Across all joining nodes, VoltDB reports the maximum difference between clocks.

In this case, VoltDB is telling you that for some pair of nodes including a joining node and the first node you started (the startup leader), the difference between the compared clocks was 120ms, which is a lot for machines on NTP that are in the same datacenter.

You can check network latency using ping between nodes, but make sure you're using the same network interface you've setup VoltDB to run on.