This was unexpected to me:
> Clock clock = Clock.systemUTC();
> Instant.now(clock).minus(3, ChronoUnit.DAYS);
java.time.Instant res4 = 2016-10-04T00:57:20.840Z
> Instant.now(clock).minus(3, ChronoUnit.YEARS);
java.time.temporal.UnsupportedTemporalTypeException: Unsupported unit: Years
As a workaround I have to do this:
> Instant.now(clock).atOffset(ZoneOffset.UTC).minus(3, ChronoUnit.YEARS).toInstant();
java.time.Instant res11 = 2013-10-07T01:02:56.361Z
I am curios why Instant does not support YEARS. Did developers just give up on it?
(In my actual code I tried to subtract a Period.ofYears(3)
but the quoted Instant methods are the ones being called in the end).
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.
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).