fix: fieldvalue gettimestamp (#279)

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Fixes #16

google-cloud-bigquery/src/main/java/com/google/cloud/bigquery/FieldValue.java

@@ -182,7 +182,9 @@ public boolean getBooleanValue() {
   public long getTimestampValue() {
     // timestamps are encoded in the format 1408452095.22 where the integer part is seconds since
     // epoch (e.g. 1408452095.22 == 2014-08-19 07:41:35.220 -05:00)
-    return new Double(Double.valueOf(getStringValue()) * MICROSECONDS).longValue();
+    BigDecimal secondsWithMicro = new BigDecimal(getStringValue());
+    BigDecimal scaled = secondsWithMicro.scaleByPowerOfTen(6);
+    return scaled.longValue();
   }
 
   /**

google-cloud-bigquery/src/test/java/com/google/cloud/bigquery/FieldValueTest.java

@@ -89,6 +89,14 @@ public void testFromPb() {
     assertEquals(FieldValue.fromPb(TIMESTAMP_FIELD), value.getRepeatedValue().get(1));
   }
 
+  @Test
+  public void testTimestamp() {
+    FieldValue fieldValue = FieldValue.of(FieldValue.Attribute.PRIMITIVE, "-1.9954383398377106E10");
+    long received = fieldValue.getTimestampValue();
+    long expected = -19954383398377106L;
+    assertEquals(expected, received);
+  }
+
   @Test
   public void testEquals() {
     FieldValue booleanValue = FieldValue.of(FieldValue.Attribute.PRIMITIVE, "false");