创建一个计时器，点击开始计时，计时器开始工作，点击结束计时，计时器停止工作，在开始计时前判断当前计时器是否已经开始工作，获取显示时间的p标签，修改页面时间

Sure, I can help you create a timer. Here is the JavaScript code:

// Get the p element to display time
let display = document.getElementById("time-display");

let startTime; // variable to store start time
let timerInterval; // setInterval variable for timer

// function to start timer
function startTimer() {
  // Check if timer already started
  if (startTime) {
    alert("Timer already started!");
    return;
  }
  // Get current time and store as start time
  startTime = new Date().getTime();
  // Call updateTimer function every second
  timerInterval = setInterval(updateTimer, 1000);
}

// function to stop timer
function stopTimer() {
  // Check if timer not started
  if (!startTime) {
    alert("Timer not started yet!");
    return;
  }
  // Clear the timer interval
  clearInterval(timerInterval);
  // Reset start time
  startTime = null;
}

// function to update timer display
function updateTimer() {
  // Get current time
  let currentTime = new Date().getTime();
  // Calculate time difference (in seconds)
  let timeDiff = (currentTime - startTime) / 1000;
  // Calculate minutes and seconds
  let minutes = Math.floor(timeDiff / 60);
  let seconds = Math.floor(timeDiff % 60);
  // Format time display (MM:SS)
  let timeDisplay = `${minutes.toString().padStart(2, "0")}:${seconds.toString().padStart(2, "0")}`;
  // Update the p element with time display
  display.textContent = timeDisplay;
}

To start the timer, call the `startTimer()` function. To stop the timer, call the `stopTimer()` function. You need to add an HTML element with `id="time-display"` to display the timer.