@Override
 public DateLogicalTimeInterval subtract(DateLogicalTimeInterval intervalToSubstract)
     throws IllegalTimeArithmetic, InvalidLogicalTimeInterval {
   return new DateLogicalTimeInterval(value.minus(intervalToSubstract.getValue()));
 }
 @Override
 public int compareTo(DateLogicalTimeInterval other) {
   return value.compareTo(other.getValue());
 }
 @Override
 public DateLogicalTimeInterval add(DateLogicalTimeInterval intervalToAdd)
     throws IllegalTimeArithmetic, InvalidLogicalTimeInterval {
   return new DateLogicalTimeInterval(value.plus(intervalToAdd.getValue()));
 }