fix: fix conversion for pre-epoch timestamps

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

@@ -119,7 +119,13 @@ public static Timestamp now() {
    * @throws IllegalArgumentException if the timestamp is outside the representable range
    */
   public static Timestamp of(java.sql.Timestamp timestamp) {
-    return ofTimeSecondsAndNanos(timestamp.getTime() / 1000, timestamp.getNanos());
+    // TODO: replace with Math.floorDiv when we drop Java 7 support
+    long secs = timestamp.getTime() / 1000;
+    int nanos = timestamp.getNanos();
+    if (secs < 0) {
+      --secs;
+    }
+    return Timestamp.ofTimeSecondsAndNanos(secs, nanos);
   }
 
   /**

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

@@ -80,6 +80,30 @@ public void ofDate() {
     assertThat(timestamp.getNanos()).isEqualTo(expectedNanos);
   }
 
+  @Test
+  public void ofSqlTimestamp() {
+    String expectedTimestamp = "1970-01-01T00:00:12.345000000Z";
+    java.sql.Timestamp input = new java.sql.Timestamp(12345);
+    Timestamp timestamp = Timestamp.of(input);
+    assertThat(timestamp.toString()).isEqualTo(expectedTimestamp);
+  }
+
+  @Test
+  public void ofSqlTimestampPreEpoch() {
+    String expectedTimestamp = "1969-12-31T23:59:47.655000000Z";
+    java.sql.Timestamp input = new java.sql.Timestamp(-12345);
+    Timestamp timestamp = Timestamp.of(input);
+    assertThat(timestamp.toString()).isEqualTo(expectedTimestamp);
+  }
+
+  @Test
+  public void ofSqlTimestampOnEpoch() {
+    String expectedTimestamp = "1970-01-01T00:00:00Z";
+    java.sql.Timestamp input = new java.sql.Timestamp(0);
+    Timestamp timestamp = Timestamp.of(input);
+    assertThat(timestamp.toString()).isEqualTo(expectedTimestamp);
+  }
+
   @Test
   public void ofDatePreEpoch() {
     Timestamp timestamp = Timestamp.of(TEST_DATE_PRE_EPOCH);