js中每周重复倒计时，困扰我的小细节

问：

我在这里找到了一个脚本，我想使用它：

var curday;
var secTime;
var ticker;
 
function getSeconds() {
 var nowDate = new Date();
 var dy = 4 ; //Sunday through Saturday, 0 to 6
 var countertime = new Date(nowDate.getFullYear(),nowDate.getMonth(),nowDate.getDate(),20,30,0); //20 out of 24 hours = 8pm
 
 var curtime = nowDate.getTime(); //current time
 var atime = countertime.getTime(); //countdown time
 var diff = parseInt((atime - curtime)/1000);
 if (diff > 0) { curday = dy - nowDate.getDay() }
 else { curday = dy - nowDate.getDay() -1 } //after countdown time
 if (curday < 0) { curday += 7; } //already after countdown time, switch to next week
 if (diff <= 0) { diff += (86400 * 7) }
 startTimer (diff);
}
 
function startTimer(secs) {
 secTime = parseInt(secs);
 ticker = setInterval("tick()",1000);
 tick(); //initial count display
}
 
function tick() {
 var secs = secTime;
 if (secs>0) {
  secTime--;
 }
 else {
  clearInterval(ticker);
  getSeconds(); //start over
 }
 
 var days = Math.floor(secs/86400);
 secs %= 86400;
 var hours= Math.floor(secs/3600);
 secs %= 3600;
 var mins = Math.floor(secs/60);
 secs %= 60;

 

 //update the time display
 document.getElementById("days").innerHTML = curday;
 document.getElementById("hours").innerHTML = ((hours < 10 ) ? "0" : "" ) + hours;
 document.getElementById("minutes").innerHTML = ( (mins < 10) ? "0" : "" ) + mins;
 document.getElementById("seconds").innerHTML = ( (secs < 10) ? "0" : "" ) + secs;


if (curday == 1) {
    document.getElementById("days_text").innerHTML = "Day"
} 


}

我了解它是如何工作的，并且可以从这里使用它，但我想更深入地了解原因：

 if (diff <= 0) { diff += (86400 * 7) }

我们增加了整整一周的秒数，而 diff 计算的唯一内容是我们在给定的一天是否超过了目标小时数。为什么它必须是整整一周，而不仅仅是 24 小时？非常感谢您的帮助。