RFR: 8368856: Add methods for saturating Duration arithmetic to Instant [v7]

[Roger Riggs]

On Wed, 8 Oct 2025 21:50:55 GMT, Pavel Rappo <prappo at openjdk.org> wrote:

>> A Period with only day units *can* be added to an Instant.  (Because the day can be converted to seconds).
>> 
>> jshell> import java.time.*
>> jshell> var i = Instant.now()
>> i ==> 2025-10-08T20:02:15.231622Z
>> jshell> var p = Period.of(0,0,1)
>> p ==> P1D
>> jshell> i.plus(p)
>> $4 ==> 2025-10-09T20:02:15.231622Z
>
> You are right: if a `Period` is carefully picked, it can be added to `Instant`. However, one cannot _always_ convert days to seconds. For example,
> 
> 
> jshell> Duration.from(Period.ofDays(1))
> |  Exception java.time.temporal.UnsupportedTemporalTypeException: Unit must not have an estimated duration
> |        at Duration.plus (Duration.java:733)
> |        at Duration.from (Duration.java:340)
> |        at (#23:1)
> 
> 
> If you ask me, that feels surprising.

As specified and implemented, `Duration.plus(long amount, TemporalUnit)` does not make an exception for amount == 0.  For any other value of month or years, the value is an estimate and the exception is thrown.
It might be useful to consider a change (as a separate enhancement)

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PR Review Comment: https://git.openjdk.org/jdk/pull/27549#discussion_r2416958704