MongoTimestamp is used by sharding. If you're not looking to write sharding tools, what you probably want is MongoDate.
MongoTimestamp is 4 bytes of timestamp (seconds since the epoch) and 4 bytes of increment.
This class is not for measuring time, creating a timestamp on a document or automatically adding or updating a timestamp on a document. Unless you are writing something that interacts with the sharding internals, stop, go directly to MongoDate, do not pass go, do not collect 200 dollars. This is not the class you are looking for.
If you are writing sharding tools, read on.