diff --git a/example/lib/countdown_timer_page.dart b/example/lib/countdown_timer_page.dart
index 716d022..d6a9dde 100644
--- a/example/lib/countdown_timer_page.dart
+++ b/example/lib/countdown_timer_page.dart
@@ -14,8 +14,8 @@ class _CountdownTimerPageState extends State<CountdownTimerPage> {
   @override
   void initState() {
     super.initState();
-    controller =
-        CountdownTimerController(endTime: endTime, onEnd: onEnd);
+    controller = CountdownTimerController(
+        endTime: endTime, onEnd: onEnd, startTime: null);
   }
 
   void onEnd() {
@@ -92,7 +92,8 @@ class _CountdownTimerPageState extends State<CountdownTimerPage> {
                     AnimatedBuilder(
                       animation: time.milliseconds!,
                       builder: (context, child) {
-                        return Text("${(time.milliseconds!.value * 1000).toInt()}");
+                        return Text(
+                            "${(time.milliseconds!.value * 1000).toInt()}");
                       },
                     )
                   ],
diff --git a/example/pubspec.lock b/example/pubspec.lock
index 07c5146..2fd9b29 100644
--- a/example/pubspec.lock
+++ b/example/pubspec.lock
@@ -5,56 +5,56 @@ packages:
     dependency: transitive
     description:
       name: async
-      url: "https://pub.flutter-io.cn"
+      url: "https://pub.dartlang.org"
     source: hosted
-    version: "2.5.0"
+    version: "2.8.2"
   boolean_selector:
     dependency: transitive
     description:
       name: boolean_selector
-      url: "https://pub.flutter-io.cn"
+      url: "https://pub.dartlang.org"
     source: hosted
     version: "2.1.0"
   characters:
     dependency: transitive
     description:
       name: characters
-      url: "https://pub.flutter-io.cn"
+      url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.1.0"
+    version: "1.2.0"
   charcode:
     dependency: transitive
     description:
       name: charcode
-      url: "https://pub.flutter-io.cn"
+      url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.2.0"
+    version: "1.3.1"
   clock:
     dependency: transitive
     description:
       name: clock
-      url: "https://pub.flutter-io.cn"
+      url: "https://pub.dartlang.org"
     source: hosted
     version: "1.1.0"
   collection:
     dependency: transitive
     description:
       name: collection
-      url: "https://pub.flutter-io.cn"
+      url: "https://pub.dartlang.org"
     source: hosted
     version: "1.15.0"
   cupertino_icons:
     dependency: "direct main"
     description:
       name: cupertino_icons
-      url: "https://pub.flutter-io.cn"
+      url: "https://pub.dartlang.org"
     source: hosted
     version: "0.1.3"
   fake_async:
     dependency: transitive
     description:
       name: fake_async
-      url: "https://pub.flutter-io.cn"
+      url: "https://pub.dartlang.org"
     source: hosted
     version: "1.2.0"
   flutter:
@@ -68,7 +68,7 @@ packages:
       path: ".."
       relative: true
     source: path
-    version: "3.0.1"
+    version: "4.1.0"
   flutter_test:
     dependency: "direct dev"
     description: flutter
@@ -78,21 +78,28 @@ packages:
     dependency: transitive
     description:
       name: matcher
-      url: "https://pub.flutter-io.cn"
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "0.12.11"
+  material_color_utilities:
+    dependency: transitive
+    description:
+      name: material_color_utilities
+      url: "https://pub.dartlang.org"
     source: hosted
-    version: "0.12.10"
+    version: "0.1.3"
   meta:
     dependency: transitive
     description:
       name: meta
-      url: "https://pub.flutter-io.cn"
+      url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.3.0"
+    version: "1.7.0"
   path:
     dependency: transitive
     description:
       name: path
-      url: "https://pub.flutter-io.cn"
+      url: "https://pub.dartlang.org"
     source: hosted
     version: "1.8.0"
   sky_engine:
@@ -104,57 +111,57 @@ packages:
     dependency: transitive
     description:
       name: source_span
-      url: "https://pub.flutter-io.cn"
+      url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.8.0"
+    version: "1.8.1"
   stack_trace:
     dependency: transitive
     description:
       name: stack_trace
-      url: "https://pub.flutter-io.cn"
+      url: "https://pub.dartlang.org"
     source: hosted
     version: "1.10.0"
   stream_channel:
     dependency: transitive
     description:
       name: stream_channel
-      url: "https://pub.flutter-io.cn"
+      url: "https://pub.dartlang.org"
     source: hosted
     version: "2.1.0"
   string_scanner:
     dependency: transitive
     description:
       name: string_scanner
-      url: "https://pub.flutter-io.cn"
+      url: "https://pub.dartlang.org"
     source: hosted
     version: "1.1.0"
   term_glyph:
     dependency: transitive
     description:
       name: term_glyph
-      url: "https://pub.flutter-io.cn"
+      url: "https://pub.dartlang.org"
     source: hosted
     version: "1.2.0"
   test_api:
     dependency: transitive
     description:
       name: test_api
-      url: "https://pub.flutter-io.cn"
+      url: "https://pub.dartlang.org"
     source: hosted
-    version: "0.2.19"
+    version: "0.4.8"
   typed_data:
     dependency: transitive
     description:
       name: typed_data
-      url: "https://pub.flutter-io.cn"
+      url: "https://pub.dartlang.org"
     source: hosted
     version: "1.3.0"
   vector_math:
     dependency: transitive
     description:
       name: vector_math
-      url: "https://pub.flutter-io.cn"
+      url: "https://pub.dartlang.org"
     source: hosted
-    version: "2.1.0"
+    version: "2.1.1"
 sdks:
-  dart: ">=2.12.0 <3.0.0"
+  dart: ">=2.14.0 <3.0.0"
diff --git a/lib/countdown_timer_controller.dart b/lib/countdown_timer_controller.dart
index e666749..c0dbbff 100644
--- a/lib/countdown_timer_controller.dart
+++ b/lib/countdown_timer_controller.dart
@@ -6,8 +6,12 @@ import 'package:flutter_countdown_timer/index.dart';
 ///Countdown timer controller.
 class CountdownTimerController extends ChangeNotifier {
   CountdownTimerController(
-      {required int endTime, this.onEnd, TickerProvider? vsync})
-      : this._endTime = endTime {
+      {required int? endTime,
+      required int? startTime,
+      this.onEnd,
+      TickerProvider? vsync})
+      : this._endTime = endTime,
+        this._startTime = startTime {
     if (vsync != null) {
       this._animationController =
           AnimationController(vsync: vsync, duration: Duration(seconds: 1));
@@ -18,7 +22,10 @@ class CountdownTimerController extends ChangeNotifier {
   final VoidCallback? onEnd;
 
   ///The end time of the countdown.
-  int _endTime;
+  int? _endTime;
+
+  ///The end time of the countdown.
+  int? _startTime;
 
   ///Is the countdown running.
   bool _isRunning = false;
@@ -74,8 +81,14 @@ class CountdownTimerController extends ChangeNotifier {
 
   ///Calculate current remaining time.
   CurrentRemainingTime? _calculateCurrentRemainingTime() {
-    int remainingTimeStamp =
-        (_endTime - DateTime.now().millisecondsSinceEpoch) ~/ 1000;
+    int remainingTimeStamp = 0;
+    if (_endTime != null)
+      remainingTimeStamp =
+          (_endTime! - DateTime.now().millisecondsSinceEpoch) ~/ 1000;
+    else
+      remainingTimeStamp =
+          (DateTime.now().millisecondsSinceEpoch - _startTime!) ~/ 1000;
+
     if (remainingTimeStamp <= 0) {
       return null;
     }
diff --git a/lib/flutter_countdown_timer.dart b/lib/flutter_countdown_timer.dart
index b285642..0617431 100644
--- a/lib/flutter_countdown_timer.dart
+++ b/lib/flutter_countdown_timer.dart
@@ -25,17 +25,21 @@ class CountdownTimer extends StatefulWidget {
   ///The end time of the countdown.
   final int? endTime;
 
-  CountdownTimer({
-    Key? key,
-    this.endWidget = const Center(
-      child: Text('The current time has expired'),
-    ),
-    this.widgetBuilder,
-    this.controller,
-    this.textStyle,
-    this.endTime,
-    this.onEnd,
-  })  : assert(endTime != null || controller != null),
+  ///The end time of the countdown.
+  final int? startTime;
+
+  CountdownTimer(
+      {Key? key,
+      this.endWidget = const Center(
+        child: Text('The current time has expired'),
+      ),
+      this.widgetBuilder,
+      this.controller,
+      this.textStyle,
+      this.endTime,
+      this.onEnd,
+      this.startTime})
+      : assert(endTime != null || controller != null || startTime != null),
         super(key: key);
 
   @override
@@ -64,7 +68,10 @@ class _CountDownState extends State<CountdownTimer> {
   ///Generate countdown controller.
   initController() {
     controller = widget.controller ??
-        CountdownTimerController(endTime: widget.endTime!, onEnd: widget.onEnd);
+        CountdownTimerController(
+            endTime: widget.endTime,
+            onEnd: widget.onEnd,
+            startTime: widget.startTime);
     if (controller.isRunning == false) {
       controller.start();
     }
@@ -79,7 +86,8 @@ class _CountDownState extends State<CountdownTimer> {
   void didUpdateWidget(CountdownTimer oldWidget) {
     super.didUpdateWidget(oldWidget);
     if (oldWidget.endTime != widget.endTime ||
-        widget.controller != oldWidget.controller) {
+        widget.controller != oldWidget.controller ||
+        widget.startTime == oldWidget.startTime) {
       controller.dispose();
       initController();
     }
diff --git a/pubspec.lock b/pubspec.lock
index ad6d1df..3317f5f 100644
--- a/pubspec.lock
+++ b/pubspec.lock
@@ -7,7 +7,7 @@ packages:
       name: async
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "2.6.1"
+    version: "2.8.2"
   boolean_selector:
     dependency: transitive
     description:
@@ -21,14 +21,14 @@ packages:
       name: characters
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.1.0"
+    version: "1.2.0"
   charcode:
     dependency: transitive
     description:
       name: charcode
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.2.0"
+    version: "1.3.1"
   clock:
     dependency: transitive
     description:
@@ -66,14 +66,21 @@ packages:
       name: matcher
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "0.12.10"
+    version: "0.12.11"
+  material_color_utilities:
+    dependency: transitive
+    description:
+      name: material_color_utilities
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "0.1.3"
   meta:
     dependency: transitive
     description:
       name: meta
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.3.0"
+    version: "1.7.0"
   path:
     dependency: transitive
     description:
@@ -127,7 +134,7 @@ packages:
       name: test_api
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "0.3.0"
+    version: "0.4.8"
   typed_data:
     dependency: transitive
     description:
@@ -141,6 +148,6 @@ packages:
       name: vector_math
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "2.1.0"
+    version: "2.1.1"
 sdks:
-  dart: ">=2.12.0 <3.0.0"
+  dart: ">=2.14.0 <3.0.0"