Java 8 LocalDate- determining the year of a yearless Feb-29 date?

Question

My colleague and I have an interesting problem. We work with an old system that only returns date data in the format ddMMM. If the day/month is in the past for the current year, then we are to assume this date applies to next year. Otherwise it applies to the current year.

So today is 4/30/2015. If the system returned records with 12MAR, then that date translates to 3/12/2016. If the date reads 07MAY, then it translates to 5/7/2015.

However, it is unclear how to determine the year for 29FEB since it is a leap year. We cannot instantiate it with a year without the possibility of it throwing an error. We relied on a try/catch when trying to create a LocalDate off it for the current year. If it catches, we assume it belongs to next year.

Is there a more kosher way to do this?

Answer 1

• Parse the value as a MonthDay, as that's what you've got.
• If the month-day is not February 29th, just handle it as normal
• If it is February 29th, you need to special-case it:
  • Determine whether the current year is a leap year with Year.isLeap(long)
  • If it is:
  • If it's currently on or before Feb 29th, then the result is Feb 29th of this year
  • If it's currently after Feb 29th, you need rules to apply - you could choose March 1st of next year or Feb 28th of next year
  • If it's not (a leap year this year)
  • If it's currently on or before Feb 28th, again you need to rules to apply, probably returning March 1st or Feb 28th of this year
  • If it's currently after Feb 28th, then the date logically belongs to next year...
    • If next year is a leap year, the result is presumably Feb 29th of next year
    • If next year is not a leap year, again you need a rule

That's hopefully outlined the three "odd" conditions you need to account for - we don't have enough information to tell you what to do in those conditions.

Answer 2

java.time

The modern way is with the java.time classes. Specifically, MonthDay in your case.

Note that you should always specify a Locale to determine the human language to use in translation of name of month.

DateTimeFormatter f = DateTimeFormatter.ofPattern( "ddMMM" , Locale.ENGLISH );

String input = "29FEB";
MonthDay md = MonthDay.parse( input , f );

You can apply this to a year to get a LocalDate object, a date-only value of year-month-day.

LocalDate

The LocalDate class represents a date-only value without time-of-day and without time zone.

A time zone is crucial in determining a date. For any given moment, the date varies around the globe by zone. For example, a few minutes after midnight in Paris France is a new day while still “yesterday” in Montréal Québec.

ZoneId z = ZoneId.of( "America/Montreal" );
LocalDate today = LocalDate.now( z );

If we are looking at February 29, check for Leap Year. If this is not a Leap Year, then you said you want to move to next year. But what if next year is also not a Leap Year? You need to keep going until you reach a Leap Year.

int yearNumber today.getYear();
LocalDate ld = null;
if( md.equals( MonthDay.of( 2 , 29 )  && ( ! Year.of( today ).isLeap() ) ) { 
    // If asking for February 29, and this is not a leap year, move to next year, per our business rule.
    … keep adding years until you find a year that *is* a leap year.
    ld = md.atYear( yearNumber + x ); 
} else {
    ld = md.atYear( yearNumber );
}

Fall back to 28th

This Question had a special business rule about jumping to the following year if the month-day is February 29 in a non-Leap Year. But for other folks be aware the default behavior in java.time is to simply fall back to February 28 when asking for the 29th a non-Leap Year. No exception is thrown.

LocalDate february28 = 
    MonthDay.of( 2 , 29 )
            .atYear( myNonLeapYearNumber );  // 29th becomes 28th.

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About java.time

The java.time framework is built into Java 8 and later. These classes supplant the troublesome old legacy date-time classes such as java.util.Date, .Calendar, & java.text.SimpleDateFormat.

The Joda-Time project, now in maintenance mode, advises migration to java.time.

To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations. Specification is JSR 310.

Where to obtain the java.time classes?

• Java SE 8 and SE 9 and later
  • Built-in.
  • Part of the standard Java API with a bundled implementation.
  • Java 9 adds some minor features and fixes.
• Java SE 6 and SE 7
  • Much of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport.
• Android
  • The ThreeTenABP project adapts ThreeTen-Backport (mentioned above) for Android specifically.
  • See How to use….

The ThreeTen-Extra project extends java.time with additional classes. This project is a proving ground for possible future additions to java.time. You may find some useful classes here such as Interval, YearWeek, YearQuarter, and more.