Consider the constructor for Entry.

class Entry {
  // ...
  constructor(metadata?: LogEntry, data?: Data) {
    this.metadata = extend(
        {
          timestamp: new Date(),
        },
        metadata);

It expects a LogEntry typed metadata value to be passed in. Historically this value has accepted a timestamp property with a JS Date value in it. You can see that internally it expects the timestamp to be a JavaScript Date as well. logging-bunyan passes in a JS Date as well.

The problem is that LogEntry type doesn't accept Date as a value for timestamp.