diff --git a/framework/CHANGELOG.md b/framework/CHANGELOG.md
index d953889..90c1b60 100644
--- a/framework/CHANGELOG.md
+++ b/framework/CHANGELOG.md
@@ -22,6 +22,7 @@ Yii Framework 2 Change Log
 - Enh #13702: Added support for PSR-3 'Logger' (klimov-paul)
 - Enh #13706: 'Profiler' layer extracted (klimov-paul)
 - Enh #15410: Added serialization abstraction layer under `yii\serialize\*` namespace (klimov-paul)
+- Enh #608: Added `yii\web\AssetConverter::$isOutdatedCallback` allowing custom check for outdated asset conversion result (klimov-paul)
 - Enh: Objects `yii\helpers\ReplaceArrayValue`, `yii\helpers\UnsetArrayValue` now support restoring after being exported with `var_export()` function (silverfire)
 - Chg: Removed methods marked as deprecated in 2.0.x (samdark)
 - Chg #8452: Packages 'captcha', 'jquery', 'rest', 'mssql' and 'oracle' have been extracted into extensions (klimov-paul)
diff --git a/framework/web/AssetConverter.php b/framework/web/AssetConverter.php
index bcc50e7..5057f21 100644
--- a/framework/web/AssetConverter.php
+++ b/framework/web/AssetConverter.php
@@ -49,6 +49,42 @@ class AssetConverter extends Component implements AssetConverterInterface
      * significantly degrade the performance.
      */
     public $forceConvert = false;
+    /**
+     * @var callable a PHP callback, which should be invoked to check whether asset conversion result is outdated.
+     * It will be invoked only if conversion target file exists and its modification time is older then the one of source file.
+     * Callback should match following signature:
+     *
+     * ```php
+     * function (string $basePath, string $sourceFile, string $targetFile, string $sourceExtension, string $targetExtension) : bool
+     * ```
+     *
+     * where $basePath is the asset source directory; $sourceFile is the asset source file path, relative to $basePath;
+     * $targetFile is the asset target file path, relative to $basePath; $sourceExtension is the source asset file extension
+     * and $targetExtension is the target asset file extension, respectively.
+     *
+     * It should return `true` is case asset should be reconverted.
+     * For example:
+     *
+     * ```php
+     * function ($basePath, $sourceFile, $targetFile, $sourceExtension, $targetExtension) {
+     *     if (YII_ENV !== 'dev') {
+     *         return false;
+     *     }
+     *
+     *     $resultModificationTime = @filemtime("$basePath/$result");
+     *     foreach (FileHelper::findFiles($basePath, ['only' => ["*.{$sourceExtension}"]]) as $filename) {
+     *         if ($resultModificationTime < @filemtime($filename)) {
+     *             return true;
+     *         }
+     *     }
+     *
+     *     return false;
+     * }
+     * ```
+     *
+     * @since 2.1.0
+     */
+    public $isOutdatedCallback;
 
 
     /**
@@ -61,11 +97,11 @@ class AssetConverter extends Component implements AssetConverterInterface
     {
         $pos = strrpos($asset, '.');
         if ($pos !== false) {
-            $ext = substr($asset, $pos + 1);
-            if (isset($this->commands[$ext])) {
-                [$ext, $command] = $this->commands[$ext];
+            $srcExt = substr($asset, $pos + 1);
+            if (isset($this->commands[$srcExt])) {
+                [$ext, $command] = $this->commands[$srcExt];
                 $result = substr($asset, 0, $pos + 1) . $ext;
-                if ($this->forceConvert || @filemtime("$basePath/$result") < @filemtime("$basePath/$asset")) {
+                if ($this->forceConvert || $this->isOutdated($basePath, $asset, $result, $srcExt, $ext)) {
                     $this->runCommand($command, $basePath, $asset, $result);
                 }
 
@@ -77,6 +113,34 @@ class AssetConverter extends Component implements AssetConverterInterface
     }
 
     /**
+     * Checks whether asset convert result is outdated, and thus should be reconverted.
+     * @param string $basePath the directory the $asset is relative to.
+     * @param string $sourceFile the asset source file path, relative to [[$basePath]].
+     * @param string $targetFile the converted asset file path, relative to [[$basePath]].
+     * @param string $sourceExtension source asset file extension.
+     * @param string $targetExtension target asset file extension.
+     * @return bool whether asset is outdated or not.
+     * @since 2.1.0
+     */
+    protected function isOutdated($basePath, $sourceFile, $targetFile, $sourceExtension, $targetExtension)
+    {
+        $resultModificationTime = @filemtime("$basePath/$targetFile");
+        if ($resultModificationTime === false || $resultModificationTime === null) {
+            return true;
+        }
+
+        if ($resultModificationTime < @filemtime("$basePath/$sourceFile")) {
+            return true;
+        }
+
+        if ($this->isOutdatedCallback === null) {
+            return false;
+        }
+
+        return call_user_func($this->isOutdatedCallback, $basePath, $sourceFile, $targetFile, $sourceExtension, $targetExtension);
+    }
+
+    /**
      * Runs a command to convert asset files.
      * @param string $command the command to run. If prefixed with an `@` it will be treated as a [path alias](guide:concept-aliases).
      * @param string $basePath asset base path and command working directory
diff --git a/tests/framework/web/AssetConverterTest.php b/tests/framework/web/AssetConverterTest.php
index 49bcd3e..5e9b509 100644
--- a/tests/framework/web/AssetConverterTest.php
+++ b/tests/framework/web/AssetConverterTest.php
@@ -20,6 +20,9 @@ class AssetConverterTest extends \yiiunit\TestCase
      */
     protected $tmpPath;
 
+    /**
+     * {@inheritdoc}
+     */
     protected function setUp()
     {
         parent::setUp();
@@ -30,6 +33,9 @@ class AssetConverterTest extends \yiiunit\TestCase
         }
     }
 
+    /**
+     * {@inheritdoc}
+     */
     protected function tearDown()
     {
         if (is_dir($this->tmpPath)) {
@@ -62,6 +68,35 @@ EOF
     /**
      * @depends testConvert
      */
+    public function testConvertOutdated()
+    {
+        $tmpPath = $this->tmpPath;
+        $srcFilename = $tmpPath . '/test.php';
+        file_put_contents($srcFilename, <<<'EOF'
+<?php
+
+echo microtime();
+EOF
+        );
+
+        $converter = new AssetConverter();
+        $converter->commands['php'] = ['txt', 'php {from} > {to}'];
+
+        $converter->convert('test.php', $tmpPath);
+        $initialConvertTime = file_get_contents($tmpPath . '/test.txt');
+
+        usleep(1);
+        $converter->convert('test.php', $tmpPath);
+        $this->assertStringEqualsFile($tmpPath . '/test.txt', $initialConvertTime);
+
+        touch($srcFilename, time() + 1000);
+        $converter->convert('test.php', $tmpPath);
+        $this->assertNotEquals($initialConvertTime, file_get_contents($tmpPath . '/test.txt'));
+    }
+
+    /**
+     * @depends testConvertOutdated
+     */
     public function testForceConvert()
     {
         $tmpPath = $this->tmpPath;
@@ -86,4 +121,37 @@ EOF
         $converter->convert('test.php', $tmpPath);
         $this->assertNotEquals($initialConvertTime, file_get_contents($tmpPath . '/test.txt'));
     }
+
+    /**
+     * @depends testConvertOutdated
+     */
+    public function testCheckOutdatedCallback()
+    {
+        $tmpPath = $this->tmpPath;
+        $srcFilename = $tmpPath . '/test.php';
+        file_put_contents($srcFilename, <<<'EOF'
+<?php
+
+echo microtime();
+EOF
+        );
+
+        $converter = new AssetConverter();
+        $converter->commands['php'] = ['txt', 'php {from} > {to}'];
+
+        $converter->convert('test.php', $tmpPath);
+        $initialConvertTime = file_get_contents($tmpPath . '/test.txt');
+
+        $converter->isOutdatedCallback = function() {
+            return false;
+        };
+        $converter->convert('test.php', $tmpPath);
+        $this->assertStringEqualsFile($tmpPath . '/test.txt', $initialConvertTime);
+
+        $converter->isOutdatedCallback = function() {
+            return true;
+        };
+        $converter->convert('test.php', $tmpPath);
+        $this->assertNotEquals($initialConvertTime, file_get_contents($tmpPath . '/test.txt'));
+    }
 }