Comment by rfoo | Hacker News Reader

Comment by rfoo | Hacker News Reader

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> Only other edge case is if someone takes a 2024-02-29 timestamp and modify the year part, without using date library to do so.

Date library does not help here, for example, both Python and Java's date library [0][1] have only "days" in their timedelta/Duration constructor. Month and year are ambiguous. What would you do if you want someone's birthday this year? The most obvious way:

    birthday: datetime.date
    birthday_this_year = birthday.replace(year=datetime.date.today().year)

is just broken. The second line throws.

[0] https://docs.python.org/3/library/datetime.html#datetime.tim...

[1] https://docs.oracle.com/javase/8/docs/api/java/time/Duration...

The Java library actually supports this case.

It may not have the convenient "years" method directly, but you can add an arbitrary unit of time quite easily

Example with duration:

    Duration.of(1, ChronoUnit.YEARS)

Example with datetime:

    today.plus(2, ChronoUnit.YEARS)

It even supports decades, centuries, eras and half days :-)

For those curious about how it handles leap years:

    var leapDay = LocalDate.of(2024, 2, 29)
    leapDay.plus(1, ChronoUnit.YEARS);
   
    // ==> 2025-02-28

    var lastYear = LocalDate.of(2023, 3, 1);
    lastYear.plus(1, ChronoUnit.YEARS);

    // ==> 2024-03-01

    var lastFeb = LocalDate.of(2023, 2, 28);
    lastFeb.plus(1, ChronoUnit.YEARS);

    // ==> 2024-02-28

    var firstFeb = LocalDate.of(2024, 2, 1);
    leapDay.with(TemporalAdjusters.lastDayOfMonth());

    // ==> 2024-02-29

Thanks! It does this according to doc:

    This method adds the specified amount to the years field in three steps:
      * Add the input years to the year field
      * Check if the resulting date would be invalid
      * Adjust the day-of-month to the last valid day if necessary

TIL. I didn't know that Java had a builtin calendar (instead of datetime) library.

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