FULL PRODUCT VERSION :
1.8.0_45-b15

ADDITIONAL OS VERSION INFORMATION :
Any OS

A DESCRIPTION OF THE PROBLEM :
This code should throw an ArithmeticException but silently overflows:

    Instant tooMuch = Instant.ofEpochSecond(Long.MAX_VALUE / 1000, 809_000_000);
    System.out.println(tooMuch.toEpochMilli()); //prints -9223372036854775807

The problem is in the code of toEpochMilli:

    public long toEpochMilli() {
        long millis = Math.multiplyExact(seconds, 1000);
        return millis + nanos / 1000_000;
    }

which should probably be:

    public long toEpochMilli() {
        long millis = Math.multiplyExact(seconds, 1000);
        return Math.addExact(millis, nanos / 1000_000);
    }

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The code should throw an ArithmeticException
ACTUAL -
The result is a negative number due to overflow

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
  public static void main(String args[]) {
    Instant tooMuch = Instant.ofEpochSecond(Long.MAX_VALUE / 1000, 809_000_000);
    System.out.println(tooMuch.toEpochMilli()); //prints -9223372036854775807
  }
---------- END SOURCE ----------