The <modified> selector has a new built-in algorithm 'lastmodified' which computes a value based upon the lastmodified time of the file.

WHATSNEW

@@ -6,6 +6,8 @@ Other changes:
  * generatekey task now supports SubjectAlternativeName during key
    generation.
 
+ * the <modified> selector has a new built-in algorithm 'lastmodified'
+   which computes a value based upon the lastmodified time of the file.
 
 Changes from Ant 1.9.12 TO Ant 1.9.13
 =====================================

manual/Types/selectors.html

@@ -742,6 +742,7 @@
                 <li> hashvalue - HashvalueAlgorithm </li>
                 <li> digest - DigestAlgorithm </li>
                 <li> checksum - ChecksumAlgorithm </li>
+                <li> lastmodified - LastModifiedAlgorithm </li>
             </ul>
         </td>
         <td valign="top" align="center"> No, defaults to <i>digest</i> </td>
@@ -869,6 +870,12 @@
           </ul>
         </td>
       </tr>
+      <tr>
+        <td valign="top"> lastmodified </td>
+        <td valign="top"> Uses the lastModified property of a file. No additional
+          configuration is required.
+        </td>
+      </tr>
       <tr><td colspan="2"><font size="+1"><b> Cache options </b></font></td></tr>
       <tr>
         <td valign="top"> propertyfile </td>

src/main/org/apache/tools/ant/types/selectors/modifiedselector/LastModifiedAlgorithm.java

@@ -0,0 +1,53 @@
+/*
+ *  Licensed to the Apache Software Foundation (ASF) under one or more
+ *  contributor license agreements.  See the NOTICE file distributed with
+ *  this work for additional information regarding copyright ownership.
+ *  The ASF licenses this file to You under the Apache License, Version 2.0
+ *  (the "License"); you may not use this file except in compliance with
+ *  the License.  You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ *
+ */
+
+package org.apache.tools.ant.types.selectors.modifiedselector;
+
+import java.io.File;
+
+/**
+ * Computes a 'timestamp' of file based on the lastModified time of that file.
+ *
+ * @version 2018-07-25
+ * @since  Ant 1.10.6 and 1.9.14
+ */
+public class LastModifiedAlgorithm implements Algorithm {
+
+    /**
+     * This algorithm doesn't need any configuration.
+     * Therefore it's always valid.
+     * @return always true
+     */
+    public boolean isValid() {
+        return true;
+    }
+
+    /**
+     * Computes a 'timestamp' for a file based on the lastModified time.
+     * @param file  The file for which the value should be computed
+     * @return the timestamp or <i>null</i> if the timestamp couldn't be computed
+     */
+    public String getValue(File file) {
+        long lastModified = file.lastModified();
+        if (lastModified == 0L) {
+            return null;
+        }
+
+        return Long.toString(lastModified);
+    }
+}

src/main/org/apache/tools/ant/types/selectors/modifiedselector/ModifiedSelector.java

@@ -321,6 +321,8 @@ public class ModifiedSelector extends BaseExtendSelector
                 algorithm = new DigestAlgorithm();
             } else if ("checksum".equals(algoName.getValue())) {
                 algorithm = new ChecksumAlgorithm();
+            } else if ("lastmodified".equals(algoName.getValue())) {
+                algorithm = new LastModifiedAlgorithm();
             }
         } else {
             if (algorithmClass != null) {
@@ -928,7 +930,7 @@ public class ModifiedSelector extends BaseExtendSelector
 
     /**
      * The enumerated type for algorithm.
-     * The values are "hashValue", "digest" and "checksum".
+     * The values are "hashValue", "digest", "checksum" and "lastmodified".
      */
     public static class AlgorithmName extends EnumeratedAttribute {
         /**
@@ -936,7 +938,7 @@ public class ModifiedSelector extends BaseExtendSelector
          * @see EnumeratedAttribute#getValues()
          */
         public String[] getValues() {
-            return new String[] {"hashvalue", "digest", "checksum"};
+            return new String[] {"hashvalue", "digest", "checksum", "lastmodified"};
         }
     }