A DateField is not related to a ChoiceField through the object hierarchy, but it obviously uses it. Don't let the name fool you though, the DateField can handle Date, Time, and Date/Time data. This specialized class is here to make entering date and/or time values easy for the developers by encapsulating several choice fields into one DateField. Each component of the date or time becomes a separate ChoiceField that can be selected and changed. Additionally, the fields act as one unit because they handle "rolling over" date or time boundaries. If you have a field with a date value in it and you scroll the day past the end of the month, the month field will advance as well.
One thing that may be confusing is that there are actually two DateField classes listed in the class list in the documentation, and that is because there are actually two separate classes. One is a standard Java class in the javax.microedition.lcdui.DateField package. The other (the one we are interested in) is...