diff --git a/x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/datafeed/extractor/scroll/ExtractedField.java b/x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/datafeed/extractor/scroll/ExtractedField.java
index ef0dffa2691..710aa017e74 100644
--- a/x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/datafeed/extractor/scroll/ExtractedField.java
+++ b/x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/datafeed/extractor/scroll/ExtractedField.java
@@ -107,7 +107,7 @@ abstract class ExtractedField {
                 value[0] = Long.parseLong((String) value[0]);
             } else if (value[0] instanceof BaseDateTime) { // script field
                 value[0] = ((BaseDateTime) value[0]).getMillis();
-            } else {
+            } else if (value[0] instanceof Long == false) { // pre-6.0 field
                 throw new IllegalStateException("Unexpected value for a time field: " + value[0].getClass());
             }
             return value;
diff --git a/x-pack/plugin/ml/src/test/java/org/elasticsearch/xpack/ml/datafeed/extractor/scroll/ExtractedFieldTests.java b/x-pack/plugin/ml/src/test/java/org/elasticsearch/xpack/ml/datafeed/extractor/scroll/ExtractedFieldTests.java
index 54afc2e707b..d2e13368d0d 100644
--- a/x-pack/plugin/ml/src/test/java/org/elasticsearch/xpack/ml/datafeed/extractor/scroll/ExtractedFieldTests.java
+++ b/x-pack/plugin/ml/src/test/java/org/elasticsearch/xpack/ml/datafeed/extractor/scroll/ExtractedFieldTests.java
@@ -13,6 +13,7 @@ import org.joda.time.DateTime;
 import java.util.Arrays;
 
 import static org.hamcrest.Matchers.equalTo;
+import static org.hamcrest.Matchers.startsWith;
 
 public class ExtractedFieldTests extends ESTestCase {
 
@@ -96,11 +97,32 @@ public class ExtractedFieldTests extends ESTestCase {
     }
 
     public void testValueGivenTimeField() {
-        SearchHit hit = new SearchHitBuilder(42).addField("time", new DateTime(123456789L)).build();
+        final long millis = randomLong();
+        final SearchHit hit = new SearchHitBuilder(randomInt()).addField("time", new DateTime(millis)).build();
+        final ExtractedField timeField = ExtractedField.newTimeField("time", ExtractedField.ExtractionMethod.DOC_VALUE);
+        assertThat(timeField.value(hit), equalTo(new Object[] { millis }));
+    }
 
-        ExtractedField timeField = ExtractedField.newTimeField("time", ExtractedField.ExtractionMethod.DOC_VALUE);
+    public void testValueGivenStringTimeField() {
+        final long millis = randomLong();
+        final SearchHit hit = new SearchHitBuilder(randomInt()).addField("time", Long.toString(millis)).build();
+        final ExtractedField timeField = ExtractedField.newTimeField("time", ExtractedField.ExtractionMethod.DOC_VALUE);
+        assertThat(timeField.value(hit), equalTo(new Object[] { millis }));
+    }
 
-        assertThat(timeField.value(hit), equalTo(new Object[] { 123456789L }));
+    public void testValueGivenPre6xTimeField() {
+        // Prior to 6.x, timestamps were simply `long` milliseconds-past-the-epoch values
+        final long millis = randomLong();
+        final SearchHit hit = new SearchHitBuilder(randomInt()).addField("time", millis).build();
+        final ExtractedField timeField = ExtractedField.newTimeField("time", ExtractedField.ExtractionMethod.DOC_VALUE);
+        assertThat(timeField.value(hit), equalTo(new Object[] { millis }));
+    }
+
+    public void testValueGivenUnknownFormatTimeField() {
+        final SearchHit hit = new SearchHitBuilder(randomInt()).addField("time", new Object()).build();
+        final ExtractedField timeField = ExtractedField.newTimeField("time", ExtractedField.ExtractionMethod.DOC_VALUE);
+        assertThat(expectThrows(IllegalStateException.class, () -> timeField.value(hit)).getMessage(),
+            startsWith("Unexpected value for a time field"));
     }
 
     public void testAliasVersusName() {
diff --git a/x-pack/plugin/ml/src/test/java/org/elasticsearch/xpack/ml/datafeed/extractor/scroll/ExtractedFieldsTests.java b/x-pack/plugin/ml/src/test/java/org/elasticsearch/xpack/ml/datafeed/extractor/scroll/ExtractedFieldsTests.java
index a921d1f3e73..7e98dd417cd 100644
--- a/x-pack/plugin/ml/src/test/java/org/elasticsearch/xpack/ml/datafeed/extractor/scroll/ExtractedFieldsTests.java
+++ b/x-pack/plugin/ml/src/test/java/org/elasticsearch/xpack/ml/datafeed/extractor/scroll/ExtractedFieldsTests.java
@@ -64,11 +64,25 @@ public class ExtractedFieldsTests extends ESTestCase {
     }
 
     public void testTimeFieldValue() {
-        SearchHit hit = new SearchHitBuilder(1).addField("time", new DateTime(1000L)).build();
+        final long millis = randomLong();
+        final SearchHit hit = new SearchHitBuilder(randomInt()).addField("time", new DateTime(millis)).build();
+        final ExtractedFields extractedFields = new ExtractedFields(timeField, Collections.singletonList(timeField));
+        assertThat(extractedFields.timeFieldValue(hit), equalTo(millis));
+    }
 
-        ExtractedFields extractedFields = new ExtractedFields(timeField, Arrays.asList(timeField));
+    public void testStringTimeFieldValue() {
+        final long millis = randomLong();
+        final SearchHit hit = new SearchHitBuilder(randomInt()).addField("time", Long.toString(millis)).build();
+        final ExtractedFields extractedFields = new ExtractedFields(timeField, Collections.singletonList(timeField));
+        assertThat(extractedFields.timeFieldValue(hit), equalTo(millis));
+    }
 
-        assertThat(extractedFields.timeFieldValue(hit), equalTo(1000L));
+    public void testPre6xTimeFieldValue() {
+        // Prior to 6.x, timestamps were simply `long` milliseconds-past-the-epoch values
+        final long millis = randomLong();
+        final SearchHit hit = new SearchHitBuilder(randomInt()).addField("time", millis).build();
+        final ExtractedFields extractedFields = new ExtractedFields(timeField, Collections.singletonList(timeField));
+        assertThat(extractedFields.timeFieldValue(hit), equalTo(millis));
     }
 
     public void testTimeFieldValueGivenEmptyArray() {