/**
 * If the entry has the attribute {@code attributeType},
 * converts the attribute value specified by {@code attributeType}
 * to a {@code Date} and returns the date; otherwise returns {@code null}.
 *
 * @param attributeType
 *   The attribute type whose value will be converted 
 *   to a {@code Date}; must not be {@code null}.
 *
 * @return attributeType's value as a {@code Date}. 
 */
    private Date getTime(String attributeType) {
      return entry.hasAttribute(attributeType)
        ? entry.getAttributeValueAsDate(attributeType)
        : null;
    }