The DATETIME data type consists of two parts, one an ABL date and one an ABL time. The unit of time is milliseconds from midnight.
The DATETIME data type is useful for dealing with datetime data in the local time of the session that assigned the data, not absolute time. It is also useful for applications that deal with datetime data from one, and only one, time zone.
An example is a procedure that determines peak-time or off-peak times for billing telephone calls. Even though the calls could be made from different time zones, it is the local time of the call that determines the cost of the call.