Calculate last third of two Dates in Java

java.time

I recommend that you use java.time, the modern Java date and time API, for your date and time work.

    ZoneId zone = ZoneId.of("America/Boise");
    ZonedDateTime zdt1 = ZonedDateTime.of(2020, 12, 26, 22, 0, 0, 0, zone);
    ZonedDateTime zdt2 = ZonedDateTime.of(2020, 12, 27, 1, 0, 0, 0, zone);
    
    Duration fullElapsedTime = Duration.between(zdt1, zdt2);
    Duration twoThirds = fullElapsedTime.multipliedBy(2).dividedBy(3);
    ZonedDateTime lastThird = zdt1.plus(twoThirds);
    
    System.out.println(lastThird);

Output from this snippet is:

2020-12-27T00:00-07:00[America/Boise]

Three things I like about this code are:

1. It pretty well mimics the way one would do the calculation by hand and how you would explain to someone else which calculation you want at all.
2. It takes any transistion to or from summer time (DST) into account.
3. It leaves the actual calculation to the library methods. It involves no low-level addition or division in your own code.

Link

Oracle tutorial: Date Time explaining how to use java.time.

One way to do this is the following:

Date midpoint = new Date((date1.getTime() + 2 * date2.getTime()) / 3);

If you insist using Date.
It should be a simple math:

1. First, need to calculate the difference between the two dates (in milliseconds).
2. Then get the 2/3 rd of that value and add it to the first date.

private static Date calculateLastThird(Date d1, Date d2) { 
        // Calculate time difference in milliseconds 
        long differenceMillis = d2.getTime() - d1.getTime();
        double millisToAdd = (2.0/3.0) * differenceMillis;
        
        Date twoThird = new Date(d1.getTime() + (long) millisToAdd);
        
        return twoThird;
    }
    
    public static void main(String args[]) {
        SimpleDateFormat sdf1 = new SimpleDateFormat("yyyy/MM/dd HH:mm:ss");
        try {
            Date date1= sdf1.parse("2020/12/26 22:00:00");
            Date date2= sdf1.parse("2020/12/27 01:00:00");
            SimpleDateFormat sdf2 = new SimpleDateFormat("HH:mm");
            System.out.println("Last third: " + sdf2.format(calculateLastThird(date1, date2)));
            
            Date date3= sdf1.parse("2020/12/26 22:25:00");
            Date date4= sdf1.parse("2020/12/27 01:45:00");
            System.out.println("Last third: " + sdf2.format(calculateLastThird(date3, date4)));
            
        } catch (ParseException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }  
    }

Sample output of above code:

Last third: 00:00
Last third: 00:38