directly display countdown

by extracting the countdown update to a dedicated function we can call it before setting the interval. thus the countdown is updated the instant it is possible instead of needing to wait for the timer interval.

main.js

@@ -5,7 +5,7 @@ const minute = second*60;
 const hour = minute*60;
 const day = hour*24;
 
-setInterval(() => {
+const update_countdown = () => {
     const now = new Date();
     const diff = target - now;
 
@@ -23,4 +23,7 @@ setInterval(() => {
 
         document.getElementById(unit).innerHTML = value;
     });
-}, 500);
+};
+
+update_countdown();
+setInterval(update_countdown, 500);