Fix Timestamps fromDate for negative 'exact second' java.sql.Timestamps

java/util/src/main/java/com/google/protobuf/util/Timestamps.java

@@ -344,7 +344,7 @@ public final class Timestamps {
     if (date instanceof java.sql.Timestamp) {
       java.sql.Timestamp sqlTimestamp = (java.sql.Timestamp) date;
       long time = sqlTimestamp.getTime();
-      long integralSeconds = (time < 0) ? time / 1000L - 1 : time / 1000L ; // truncate the fractional seconds
+      long integralSeconds = (time < 0 && time % 1000 != 0) ? time / 1000L - 1 : time / 1000L ; // truncate the fractional seconds
       return Timestamp.newBuilder()
           .setSeconds(integralSeconds)
           .setNanos(sqlTimestamp.getNanos())

java/util/src/test/java/com/google/protobuf/util/TimestampsTest.java

@@ -483,6 +483,13 @@ public class TimestampsTest {
     assertThat(Timestamps.toString(timestamp)).isEqualTo("1969-12-31T23:59:58.889Z");
   }
 
+  @Test
+  public void testFromSqlTimestamp_beforeEpochWholeSecond() {
+    Date date = new java.sql.Timestamp(-2000);
+    Timestamp timestamp = Timestamps.fromDate(date);
+    assertThat(Timestamps.toString(timestamp)).isEqualTo("1969-12-31T23:59:58Z");
+  }
+
   @Test
   public void testTimeOperations() throws Exception {
     Timestamp start = Timestamps.parse("0001-01-01T00:00:00Z");