Custom Date-Time fields can be added by clicking on the Plus button in the Field manager, on both Organization and Event levels, and selecting the option Date-Time in the dropdown offered.
Planners can edit the name of the field (e.i. Date and time of birth), label for the field visible to contacts and participants (which may or may not be the same as the name of the field) and the category the field belongs to (e.i. Personal information).
The field can be further customized by selecting what might be the default values for this field - the default date and time.