Why Instant does not support operations with ChronoUnit.YEARS?

I guess it happens because Instant does not contain information about time zone. It means that same Instant can be interpreted as different date-time value in different time zones. Let's assume we have Instant which is is represented as 2016.01.01 00:30:00 in, let's say, UTC+2 time zone. The same Instant means 2015.12.31 23:30:00 in UTC+1 time zone. 2016 is a leap year, it's length is 366 days, so in order to get Instant minus 1 year, we have to subtract 366 days from it. But 2015 is not a leap year, it's length is 365 days, so we have to subtract 365 days from Instant. This ambiguity causes the fact that Instant does not support ChronoUnit.YEARS. Similar issue causes Instant to not support ChronoUnit.MONTHS. And probably absence of DST information causes Instant to not support ChronoUnit.WEEKS.

I'm taking a stab at it in what looks to me like something very logical.

Here is the code for the method plus(long, TemporalUnit) (which is used in minus(...)):

@Override
public Instant plus(long amountToAdd, TemporalUnit unit) {
    if (unit instanceof ChronoUnit) {
        switch ((ChronoUnit) unit) {
            case NANOS: return plusNanos(amountToAdd);
            case MICROS: return plus(amountToAdd / 1000_000, (amountToAdd % 1000_000) * 1000);
            case MILLIS: return plusMillis(amountToAdd);
            case SECONDS: return plusSeconds(amountToAdd);
            case MINUTES: return plusSeconds(Math.multiplyExact(amountToAdd, SECONDS_PER_MINUTE));
            case HOURS: return plusSeconds(Math.multiplyExact(amountToAdd, SECONDS_PER_HOUR));
            case HALF_DAYS: return plusSeconds(Math.multiplyExact(amountToAdd, SECONDS_PER_DAY / 2));
            case DAYS: return plusSeconds(Math.multiplyExact(amountToAdd, SECONDS_PER_DAY));
        }
        throw new UnsupportedTemporalTypeException("Unsupported unit: " + unit);
    }
    return unit.addTo(this, amountToAdd);
}

We can see that the results are calculated by multiplying seconds representation of units, a year cannot be logically and consistently represented by seconds for obvious reasons.

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Addition

I can see another obvious reason why : constants used in the method above come from java.time.LocalTime. The constants only define units up to days. No constant above days are defined (in LocalDate and LocalDateTime neither).