Fix the error when calling Timestamp.of(Date date) when date is pre epoch (#3434)

* Fix the error when calling Timestamp.of(Date date) when date is pre
epoch.

Author: yihanzhen

google-cloud-clients/google-cloud-core/src/main/java/com/google/cloud/Timestamp.java

@@ -78,9 +78,12 @@ public static Timestamp ofTimeSecondsAndNanos(long seconds, int nanos) {
    * @throws IllegalArgumentException if the timestamp is outside the representable range
    */
   public static Timestamp ofTimeMicroseconds(long microseconds) {
-    long seconds = TimeUnit.MICROSECONDS.toSeconds(microseconds);
-    int nanos =
-        (int) TimeUnit.MICROSECONDS.toNanos(microseconds - TimeUnit.SECONDS.toMicros(seconds));
+    long seconds = microseconds / 1_000_000;
+    int nanos = (int)(microseconds % 1_000_000 * 1000);
+    if (nanos < 0) {
+      seconds--;
+      nanos += 1_000_000_000;
+    }
     checkArgument(
         Timestamps.isValid(seconds, nanos), "timestamp out of range: %s, %s", seconds, nanos);
     return new Timestamp(seconds, nanos);

google-cloud-clients/google-cloud-core/src/test/java/com/google/cloud/TimestampTest.java

@@ -39,7 +39,9 @@ public class TimestampTest {
   private static final long TEST_TIME_MICROSECONDS = 10000100L;
   private static final long TEST_TIME_MILLISECONDS =
       TimeUnit.SECONDS.toMillis(1444662894L) + TimeUnit.MICROSECONDS.toMillis(1234);
+  private static final long TEST_TIME_MILLISECONDS_NEGATIVE = -1000L;
   private static final Date TEST_DATE = new Date(TEST_TIME_MILLISECONDS);
+  private static final Date TEST_DATE_PRE_EPOCH = new Date(TEST_TIME_MILLISECONDS_NEGATIVE);
 
   @Rule public ExpectedException expectedException = ExpectedException.none();
 
@@ -80,6 +82,19 @@ public void ofDate() {
     assertThat(timestamp.getNanos()).isEqualTo(expectedNanos);
   }
 
+  @Test
+  public void ofDatePreEpoch() {
+    Timestamp timestamp = Timestamp.of(TEST_DATE_PRE_EPOCH);
+    long expectedSeconds = TEST_TIME_MILLISECONDS_NEGATIVE / 1_000;
+    int expectedNanos = (int)(TEST_TIME_MILLISECONDS_NEGATIVE % 1_000 * 1000_000);
+    if (expectedNanos < 0) {
+      expectedSeconds--;
+      expectedNanos += 1_000_000_000;
+    }
+    assertThat(timestamp.getSeconds()).isEqualTo(expectedSeconds);
+    assertThat(timestamp.getNanos()).isEqualTo(expectedNanos);
+  }
+
   @Test
   public void toDate() {
     Timestamp timestamp = Timestamp.ofTimeSecondsAndNanos(TEST_TIME_SECONDS, 1234 * 1000);