fix: fix conversion for pre-epoch timestamps (#160)

* test: failing test for parsing a negative timestamp

* fix: fix conversion for pre-epoch timestamps

* chore: add comment, rename test variable names

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

@@ -119,7 +119,19 @@ 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());
+    int nanos = timestamp.getNanos();
+
+    // A pre-epoch timestamp will be off by one second because of the way that integer division
+    // works. For example, -1001 / 1000 == -1. In this case of timestamps, we want this result to be
+    // -2. This causes any pre-epoch timestamp to be off by 1 second - fix this by adjusting the
+    // seconds value by 1 if the timestamp < 0.
+    // TODO: replace with Math.floorDiv when we drop Java 7 support
+    long seconds = timestamp.getTime() / 1000;
+    if (seconds < 0) {
+      --seconds;
+    }
+
+    return Timestamp.ofTimeSecondsAndNanos(seconds, 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 expectedTimestampString = "1970-01-01T00:00:12.345000000Z";
+    java.sql.Timestamp input = new java.sql.Timestamp(12345);
+    Timestamp timestamp = Timestamp.of(input);
+    assertThat(timestamp.toString()).isEqualTo(expectedTimestampString);
+  }
+
+  @Test
+  public void ofSqlTimestampPreEpoch() {
+    String expectedTimestampString = "1969-12-31T23:59:47.655000000Z";
+    java.sql.Timestamp input = new java.sql.Timestamp(-12345);
+    Timestamp timestamp = Timestamp.of(input);
+    assertThat(timestamp.toString()).isEqualTo(expectedTimestampString);
+  }
+
+  @Test
+  public void ofSqlTimestampOnEpoch() {
+    String expectedTimestampString = "1970-01-01T00:00:00Z";
+    java.sql.Timestamp input = new java.sql.Timestamp(0);
+    Timestamp timestamp = Timestamp.of(input);
+    assertThat(timestamp.toString()).isEqualTo(expectedTimestampString);
+  }
+
   @Test
   public void ofDatePreEpoch() {
     Timestamp timestamp = Timestamp.of(TEST_DATE_PRE_EPOCH);