The current_datetime_timestamp extension makes Dataset#current_datetime return an object that operates like Sequel.datetime_class.now, but will be literalized as CURRENT_TIMESTAMP.
This allows you to use the defaults_setter, timestamps, and touch model plugins and make sure that CURRENT_TIMESTAMP is used instead of a literalized timestamp value.
The reason that CURRENT_TIMESTAMP is better than a literalized version of the timestamp is that it obeys correct transactional semantics (all calls to CURRENT_TIMESTAMP in the same transaction return the same timestamp, at least on some databases).
To have current_datetime be literalized as CURRENT_TIMESTAMP for a single dataset:
ds = ds.extension(:current_datetime_timestamp)
To have current_datetime be literalized as CURRENT_TIMESTAMP for all datasets of a given database.