No need to store the position of DateField and TimeField, because this
will not change if we choose another type. The only thing what changes
is the size of the DateField, because both "DateTime" and "Date" use
this field. And for this size we just rely on the size of m_aTimeField,
because it's the same as m_aDateField (for type DateTime).

Change-Id: Ic590c62d82d8f90576479e10be9d422326032d28