All values are supplied by the client, including the 'timestamp'. This means that clocks on the clients should be synchronized (in the Cassandra server environment is useful also), as these timestamps are used for conflict resolution. In many cases the 'timestamp' is not used in client applications, and it becomes convenient to think of a column as a name/value pair. For the remainder of this document, 'timestamps' will be elided for readability. It is also worth noting the name and value are binary values, although in many applications they are UTF8 serialized strings.
Timestamps can be anything you like, but microseconds since 1970 is a convention. Whatever you use, it must be consistent across the application, otherwise earlier changes may overwrite newer ones.
what kind of Concurrency Control Method is used in Cassandra? I found out so far
that it's not done with the MVCC Method and that no vector clocks are being used.