code: public java.util.Date getDateWithZone(ZoneId timezone) {
          LocalDate pickerDate = parentDatePicker.getDate();
          if (pickerDate == null || timezone == null) {
              return null;
          }
          Instant instant = pickerDate.atStartOfDay(timezone).toInstant();
          // for backport: java.util.Date javaUtilDate = DateTimeUtils.toDate(instant);
          // for java.time: java.util.Date javaUtilDate = java.util.Date.from(instant);
          // This function was created to be compatible with both libraries.
          java.util.Date javaUtilDate = getJavaUtilDateFromInstant(instant);
          return javaUtilDate;
      }
docs: getDateWithZone, Returns the date picker value as a java.util.Date that was created using the
  specified time zone, or returns null. This will return null if either the date picker has no
  value, or if the supplied time zone was null.