diff --git a/app/App.js b/app/App.js
index 6342fa4..20e03fc 100644
--- a/app/App.js
+++ b/app/App.js
@@ -26,7 +26,7 @@ function MyTimer({ expiryTimestamp }) {
       <button onClick={resume}>Resume</button>
       <button onClick={() => {
         // Restarts to 5 minutes timer
-        var t = new Date();
+        const t = new Date();
         t.setSeconds(t.getSeconds() + 300);
         restart(t)
       }}>restart</button>
@@ -35,7 +35,7 @@ function MyTimer({ expiryTimestamp }) {
 }
 
 export default function App() {
-  var t = new Date();
+  const t = new Date();
   t.setSeconds(t.getSeconds() + 600); // 10 minutes timer
   return (
     <div>
diff --git a/src/useTimer.js b/src/useTimer.js
index 0480e80..c6d5209 100644
--- a/src/useTimer.js
+++ b/src/useTimer.js
@@ -135,6 +135,26 @@ export default function useTimer(settings) {
     setExpiryTimestamp(newExpiryTimestamp);
   }
 
+
+  // Timer expiry date calculation
+  function calculateExpiryDate() {
+    const now = new Date().getTime();
+    const distance = expiryTimestamp - now;
+    const days = Math.floor(distance / (1000 * 60 * 60 * 24));
+    const hours = Math.floor((distance % (1000 * 60 * 60 * 24)) / (1000 * 60 * 60));
+    const minutes = Math.floor((distance % (1000 * 60 * 60)) / (1000 * 60));
+    const seconds = Math.floor((distance % (1000 * 60)) / 1000);
+    if(seconds < 0) {
+      reset();
+      isValidOnExpire(onExpire) && onExpire();
+    } else {
+      setSeconds(seconds);
+      setMinutes(minutes);
+      setHours(hours);
+      setDays(days);
+    }
+  }
+
   // didMount effect
   useEffect(() => {
     start();