I need the number of days in a year and I wanted to use Java's new time API.

However, I can't do Duration.ofDays(365) because it doesn't account for leap years. And Duration.of(1, ChronoUnit.YEARS) doesn't fly because of java.time.temporal.UnsupportedTemporalTypeException: Unit must not have an estimated duration

I looked into Period, but it doesn't appear useful for going from years to days.

I feel like I'm missing something here? I could write something to add a day if the year is a leap year, but it seems like I should be able to handle this out of the box.

Short answer

No, Duration is for 1 day or less, Period is what you should use for 1 day or more.

Long answer

As per the response in Getting Duration using the new dateTime API you should be using

Period p = Period.ofYears(1);

It's important to understand the difference between Duration (exact number of nanoseconds < 1 day) and Period (variable > 1 day).

Duration won't account for leap days, daylight savings time or leap seconds, for example, and is intended for durations of less than a day, at most a few days.

So you should use Period instead.

Because different years have different number of days, if you want to find the number of days in a year, you need to specify which year you're talking about.

If you want the number of days in a specific year, you can use

Year.of(year).length()

If you want the date one year from now, you can use

LocalDate.now().plusYears(1)

or

LocalDate.now().plus(Period.ofYears(1))

If you need the number of days between two dates, you can use

ChronoUnit.DAYS.between(start, end)

So to find the number of days to the date a year from now, you can use

LocalDate today = LocalDate.now();
long days = ChronoUnit.DAYS.between(today, today.plusYears(1));

If you want to see whether a membership of one year is still valid, you can use

Period membershipLength = Period.ofYears(1);
LocalDate membershipStart = ...;
LocalDate membershipEnd = membershipStart.plus(membershipLength);

LocalDate today = LocalDate.now();
boolean memberShipEnded = today.isAfter(membershipEnd);
boolean membershipValid = !membershipEnded;