public class Period {
 private final Date startDate;
 private final Date endDate;
 //returns null if the given String
 //does not represent a valid Date
 private Date parseDate(String date) {
  ...
 }
 public Period(String start, String end) {
  startDate = parseDate(start);
  endDate = parseDate(end);
 }
 public boolean isValid() {
  return startDate!=null && endDate!=null;
 }
 public Date getStartDate() {
  if (startDate==null) 
   throw new IllegalStateException();
  return startDate;
 }
 public Date getEndDate() {
  if (endDate==null)
   throw new IllegalStateException();
  return endDate;
 }
}

shared class Period(String start, String end) {
 //returns null if the given String
 //does not represent a valid Date
 Date? parseDate(String date) => ... ;
 value maybeStartDate = parseDate(start);
 value maybeEndDate = parseDate(end);
 shared Boolean valid
  => maybeStartDate exists 
  && maybeEndDate exists;
 shared Date startDate {
  assert (exists maybeStartDate);
  return maybeStartDate;
 }
 shared Date endDate {
  assert (exists maybeEndDate);
  return maybeEndDate;
 }
}

public class Period {
 private final Date startDate;
 private final Date endDate;
 //throws if the given String
 //does not represent a valid Date
 private Date parseDate(String date)
   throws DateFormatException {
  ...
 }
 public Period(String start, String end) 
   throws DateFormatException {
  startDate = parseDate(start);
  endDate = parseDate(end);
 }
 public Date getStartDate() {
  return startDate;
 }
 public Date getEndDate() {
  return endDate;
 }
}

try {
 Period p = new Period(start, end);
 ...
}
catch (DateFormatException dfe) {
 ...
}

//returns DateFormatError if the given 
//String does not represent a valid Date
Date|DateFormatError parseDate(String date) => ... ;
shared Period|DateFormatError parsePeriod
  (String start, String end) {
 value startDate = parseDate(start);
 if (is DateFormatError startDate) {
  return startDate;
 }
 value endDate = parseDate(end);
 if (is DateFormatError endDate) {
  return endDate;
 }
 return Period(startDate, endDate);
}
shared class Period(startDate, endDate) {
 shared Date startDate;
 shared Date endDate;
}

value p = parsePeriod(start, end);
if (is DateFormatError p) {
 ...
}
else {
 ...
}

//returns null if the given String 
//does not represent a valid Date
Date? parseDate(String date) => ... ;
shared Period? parsePeriod(String start, String end)
 => if (exists startDate = parseDate(start), 
   exists endDate = parseDate(end))
  then Period(startDate, endDate)
  else null;
shared class Period(startDate, endDate) {
 shared Date startDate;
 shared Date endDate;
}