new Document("a", new Date(1483228800000L)).toJson() // same with JsonMode.STRICT

produces

{ "a" : { "$date" : 1483228800000 } }

but

{ "a" : { "$date" : "2017-01-01T00:00:00.000Z" } }

or something similar expected.

Date is serialized to format which differs with described in docs and used by mongoexport tools.

If such behavior is expected and desired, than it should be at least mentioned in docs. It seems to be consistent through all 3.x drivers. Though mongoimport works fine with format produced by Document.toJson with current tools.