I ran into an interesting situation when saving dated to MongoDB using the Mongo .Net driver (version 2.0).
When saving a date with unspecified kind, the driver converts the date to a universal datetime. It appears that the driver will assume that a DateTime object with an unspecified kind is a local time, and convert it to universal time (UTC) before committing it to the database.
In my particular case, my origin date was UTC and was read from SQL server.