edit1

src/App.js

 import React, { Component } from 'react';
 
-const App = () => {
+const cityList = [
+  {cityName: "Seoul", timezone: "Asia/Seoul"},
+  {cityName: "London", timezone: "Europe/London"},
+  {cityName: "NewYork", timezone: "America/New_York"},
+];
+
+const TimeButton = (props) => {
+  let [time,setTime] = useState(null);
+  const displayTime = () => {
+    let date = new Date();
+    setTime(date.toLocaleString("ko-KR", { timeZone: props.tz }));
+  };
+
+  return (
+    <>
+    <button onClick={displayTime} tz={props.tz}>{props.name}</button>
+    <div id='time'>{time}</div>
+    </>
+  );
+}
+
+const App = (props) => {
+    let city = cityList.map((value => {
+      console.log(value.cityName);
+      return <TimeButton name={value.cityName} tz={value.timezone} />;
+    }));
+
     return (
+      <>
       <h1>
         What time is it?
       </h1>
+      </>
     );
 }