PHP and Java functions returning different dates when calculating 6 months ahead

I have the following code to calculate what day it will be in 6 months from today.

// Java code
Date currentDate = (new SimpleDateFormat("yyyy-MM-dd")).parse("2024-08-30");
        
Calendar calendar = Calendar.getInstance();
calendar.setTime(currentDate);
calendar.add(Calendar.MONTH, 6);
Date sixMonthsLaterDate = calendar.getTime();
String sixMonthsLaterDateString = new SimpleDateFormat("yyyy-MM-dd").format(sixMonthsLaterDate);

System.out.println("sixMonthsLaterDateString: " + sixMonthsLaterDateString); // returns 2025-02-28

in Java, it returns "2025-02-28"

// PHP code
$currentDate = date_create_from_format('Y-m-d', '2024-08-30');
$sixMonthsLaterDate = $currentDate->modify('+6 month');
$sixMonthsLaterDateString = date_format($sixMonthsLaterDate, 'Y-m-d');
echo "sixMonthsLaterDateString: $sixMonthsLaterDateString"; // returns 2025-03-02

in PHP, it returns "2025-03-02"

Why are they different? Can anyone explain it? Thanks!

Solution

tl;dr

LocalDate.parse( "2024-08-30" ).plusMonths( 6 )  // Accounts for calendar months, adjusting to end-of-month if needed.

See this code run at Ideone.com.

2025-02-28

Avoid legacy date-time classes

You are using terribly flawed date-time classes that are now legacy. They were supplanted years ago by the modern java.time classes defined in JSR 310.

`LocalDate`

For a date-only value, use java.time.LocalDate.

LocalDate ld = LocalDate.parse( "2024-08-30" ) ;

Specify six months with Period class.

Period period = Period.ofMonths( 6 ) ;

Add.

LocalDate sixMonthsLater = ld.plus( period ) ;

Explain difference from PHP

As discussed in Comments, counting months is a tricky subject. Months have different lengths of 28, 29, 30, and 31 days in the modern era in the ISO 8601 calendar system.

Java approach

The java.time framework tries to account for the calendar month rather than adding an arbitrary number of days such as ( 6 * 30 ).

The algorithm is described in the Javadoc of LocalDate#plusMonths. To quote:

… three steps:

Add the input months to the month-of-year field

Check if the resulting date would be invalid

Adjust the day-of-month to the last valid day if necessary

In the case of your example input 2024-08-30, the sixth month after August is February. Then we consider the day-of-month. But there is no day 30 in any February. Nor is there a day 29 in February of that year. The last valid day of that February month is 28. Voilà, 2025-02-28 is the solution.

PHP approach

I have no idea what your PHP library is doing. It is not accounting for calendar month. It is not adding 6 * 30 days, nor 6 * 31 days.

This Comment by C3roe gives an explanation for the PHP behavior, but I’ve not researched it. To quote:

… PHP has landed on 2025-02-30, and then tried to correct the "overflow" by moving the corresponding number of days into the next month

In that approach, the two days of “overflow” is the non-existent 29 and 30. Adding those two days to February 28 of 2025 gets you to March 2.

If that is indeed the algorithm of the PHP approach, it seems bizarre to me. This would be a bastardized hybrid approach, neither fully accounting for calendar months nor precisely a count of days. This approach addresses “extra” days in the last month (February in this case), but ignores the variation in the length of the intervening months (September-January, in this case).

The correct solution

We have seen these approaches to moving forward/backward in time by months:

An arbitrary count of days, such as ( 6 * 30 ).
A count of calendar months, with adjustment to the end of a calendar month, if needed (the java.time approach).
A hybrid approach where we move over calendar months while ignoring their length, but then account for possible “overflow” days in the last month (the PHP approach, seemingly).

Other approaches may exist as well. So which is the correct approach to use?

The correct approach is… whatever the stackholders in your app project say it is. You should always raise such date-time issues with the domain experts in your project. That might be shipping agents, logistics coördinators, accountants, etc. Have them specify how month counting should operate. And have them do so in writing, I would recommend. Then copy those rules into in the comments of your codebase.