Created Reset Functionality for useTimer.

Author: IzzleNizzle

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 +24,7 @@ function MyTimer({ expiryTimestamp }) {
       <button onClick={start}>Start</button>
       <button onClick={pause}>Pause</button>
       <button onClick={resume}>Resume</button>
+      <button onClick={() => restart(500) /* resets to 50 seconds */}>restart</button>
     </div>
   );
 }

src/useTimer.js

@@ -105,7 +105,7 @@ export function useStopwatch(settings) {
 /* ---------------------- useTimer --------------------- */
 
 export function useTimer(settings) {
-  const { expiryTimestamp, onExpire } = settings || {};
+  let { expiryTimestamp, onExpire } = settings || {};
 
   const [seconds, setSeconds] = useState(0);
   function subtractSecond() {
@@ -164,7 +164,7 @@ export function useTimer(settings) {
   function start() {
     if(isValidExpiryTimestamp(expiryTimestamp) && !intervalRef.current) {
       calculateExpiryDate();
-      intervalRef.current = setInterval(() => calculateExpiryDate(), 1000);
+      intervalRef.current = setInterval(() => subtractSecond(), 1000);
     }
   }
 
@@ -192,6 +192,19 @@ export function useTimer(settings) {
     }
   }
 
+  function restart(offset) {
+    reset();
+    // Calculate fresh expiry and start timer again with new expiry ammount
+    calcNewExpiry(offset);
+    start();
+  }
+
+  function calcNewExpiry(offset) {
+    let t = new Date();
+    t.setSeconds(t.getSeconds() + offset);
+    expiryTimestamp = t;
+  }
+
   // Timer expiry date calculation
   function calculateExpiryDate() {
     var now = new Date().getTime();
@@ -236,7 +249,7 @@ export function useTimer(settings) {
     return isValid;
   }
 
-  return { seconds, minutes, hours, days, start, pause, resume };
+  return { seconds, minutes, hours, days, start, pause, resume, restart };
 }