Depois de pesquisar um pouco, encontrei estes posts:http://alexmg.com/post/2011/09/30/DateTime-precision-with-MongoDB-and-the-C-Driver.aspx

http://www.mongodb.org/display /DOCS/CSharp+Driver+Serialization+Tutorial#CSharpDriverSerializationTutorial-DateTimeSerializationOptions

Eu precisava marcar a propriedade DateTime com o atributo [BsonDateTimeOptions(Kind =DateTimeKind.Local)]