Why Instant does not support operations with ChronoUnit.YEARS?

Question

This was unexpected to me:

> Clock clock = Clock.systemUTC();

> Instant.now(clock).minus(3, ChronoUnit.DAYS);
java.time.Instant res4 = 2016-10-04T00:57:20         


        

Answer 1

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).