RubyTime opPlusNanos maybe

Time.java

public class Time {
    
    private final long timeInMillis;
    
    private final long nsec;
    
    public Time(long timeInMillis, long nsec) {
        this.timeInMillis = timeInMillis;
        this.nsec = nsec;
    }
    
    @Override
    public String toString() {
        return "" + timeInMillis + " ms, " + nsec + " ns";
    }
    
    public long getTimeInMillis() {
        return timeInMillis;
    }
    
    public Time origPlusNanos(long adjustNanos) {
        double currentNanos = getTimeInMillis() * 1000000 + nsec;

        double newNanos = currentNanos + adjustNanos;
        double newMillisPart = newNanos / 1000000;
        double newNanosPart = newNanos % 1000000;

        return new Time((long)newMillisPart, (long)newNanosPart);
    }

    public Time modPlusNanos(long adjustNanos) {
        long adjustMillis = adjustNanos / 1000000;
        double currentNanos = getTimeInMillis() * 1000000 + nsec;
        double newNanos = currentNanos + adjustNanos;
        long newMillisPart = getTimeInMillis() + adjustMillis;
        double newNanosPart = newNanos % 1000000;

        return new Time(newMillisPart, (long)newNanosPart);
    }


    private static final long NANOS_PER_MILLI = 1000000;

    public Time myPlusNanos(long adjustNanos) {
        // split the argument into nanos and millis parts
        long splitNanos = adjustNanos % NANOS_PER_MILLI;
        long splitMillis = adjustNanos / NANOS_PER_MILLI;
        
        if (splitNanos < 0) {
            splitNanos += NANOS_PER_MILLI;
            splitMillis -= 1;
        }
        
        final long newNanosPart = (splitNanos + nsec) % NANOS_PER_MILLI;
        final long overflowMillis = (splitNanos + nsec) / NANOS_PER_MILLI;
        
        final long newMillisPart = splitMillis + overflowMillis + getTimeInMillis();

        return new Time(newMillisPart, newNanosPart);
    }

}

TimeTest.java

public class TimeTest {
    
    public static Time adjust(Time time, long adjustNanos) {
        return time.myPlusNanos(adjustNanos);
    }
    
  @Test
  public void test() {
      {
          Time time = new Time(0, 0);
          assertEquals("0 ms, 1 ns", adjust(time, 1).toString());
          assertEquals("0 ms, 100 ns", adjust(time, 100).toString());
          assertEquals("0 ms, 100000 ns", adjust(time, 100000).toString());
          assertEquals("1 ms, 0 ns", adjust(time, 1000000).toString());
      }

      {
          Time time = new Time(0, 999999);
          assertEquals("1 ms, 0 ns", adjust(time, 1).toString());
          assertEquals("1 ms, 99 ns", adjust(time, 100).toString());
          assertEquals("1 ms, 99999 ns", adjust(time, 100000).toString());
          assertEquals("1 ms, 999999 ns", adjust(time, 1000000).toString());
      }

      {
          Time time = new Time(1000000000000L, 0);
          assertEquals("1000000000000 ms, 1 ns", adjust(time, 1).toString());
          assertEquals("1000000000000 ms, 100 ns", adjust(time, 100).toString());
          assertEquals("1000000000000 ms, 100000 ns", adjust(time, 100000).toString());
          assertEquals("1000000000001 ms, 0 ns", adjust(time, 1000000).toString());
      }

      {
          Time time = new Time(1, 1);
          assertEquals("0 ms, 1 ns", adjust(time, -1000000).toString());
          assertEquals("1 ms, 0 ns", adjust(time, -1).toString());
          assertEquals("0 ms, 999999 ns", adjust(time, -2).toString());
      }

  }

}