public void SerializeDates()
{
    var now = DateTime.Parse("2018-04-01T23:43:24.1234567-04:00", CultureInfo.InvariantCulture, DateTimeStyles.AdjustToUniversal | DateTimeStyles.AssumeUniversal);
    var obj = new {now};
    var json = Newtonsoft.Json.JsonConvert.SerializeObject(obj);
    var bsonFromJson = BsonDocument.Parse(json);
    var bson = obj.ToBsonDocument();
    Console.WriteLine("JSON          : {0}", json);          // prints {"now":"2018-04-02T03:43:24.1234567Z"}
    Console.WriteLine("BSON from JSON: {0}", bsonFromJson);  // prints { "now" : "2018-04-02T03:43:24.1234567Z" }
    Console.WriteLine("BSON          : {0}", bson);          // prints { "now" : ISODate("2018-04-02T03:43:24.123Z") }
}