Get current date time in Firebird

There is many way to get the current date time from the Firebird database, here are the example:

select current_date, current_time, current_timestamp, cast('now' as timestamp)
from rdb$database

The last one that use special keyword (date/time literal) called 'now' that can be cast to timestamp, date, or time. Now the question is, how to get the milisecond in SQL statement? I still don't know, maybe some people arround here can can share to us the answer?