Created Reset Functionality for useTimer.

src/App.js

@@ -9,7 +9,8 @@ function MyTimer({ expiryTimestamp }) {
     days,
     start,
     pause,
-    resume
+    resume,
+    restart
   } = useTimer({ expiryTimestamp, onExpire: () => console.warn('onExpire called') });
 
 
@@ -23,6 +24,12 @@ function MyTimer({ expiryTimestamp }) {
       <button onClick={start}>Start</button>
       <button onClick={pause}>Pause</button>
       <button onClick={resume}>Resume</button>
+      <button onClick={() => {
+        // Resets to 10 minutes timer 
+        var t = new Date();
+        t.setSeconds(t.getSeconds() + 600);
+        restart(t)
+      }}>restart</button>
     </div>
   );
 }

src/useTimer.js

@@ -105,7 +105,8 @@ export function useStopwatch(settings) {
 /* ---------------------- useTimer --------------------- */
 
 export function useTimer(settings) {
-  const { expiryTimestamp, onExpire } = settings || {};
+  const { expiryTimestamp: expiry, onExpire } = settings || {};
+  const [expiryTimestamp, setExpiryTimestamp] = useState(expiry);
 
   const [seconds, setSeconds] = useState(0);
   function subtractSecond() {
@@ -164,7 +165,7 @@ export function useTimer(settings) {
   function start() {
     if(isValidExpiryTimestamp(expiryTimestamp) && !intervalRef.current) {
       calculateExpiryDate();
-      intervalRef.current = setInterval(() => calculateExpiryDate(), 1000);
+      intervalRef.current = setInterval(() => subtractSecond(), 1000);
     }
   }
 
@@ -192,6 +193,12 @@ export function useTimer(settings) {
     }
   }
 
+  function restart(newExpiryTimestamp) {
+    reset();
+    setExpiryTimestamp(newExpiryTimestamp);
+  }
+
+
   // Timer expiry date calculation
   function calculateExpiryDate() {
     var now = new Date().getTime();
@@ -215,7 +222,7 @@ export function useTimer(settings) {
   useEffect(() => {
     start();
     return reset;
-  },[]);
+  },[expiryTimestamp]);
 
 
   // Validate expiryTimestamp
@@ -236,7 +243,7 @@ export function useTimer(settings) {
     return isValid;
   }
 
-  return { seconds, minutes, hours, days, start, pause, resume };
+  return { seconds, minutes, hours, days, start, pause, resume, restart };
 }