JS script to autocomplete start_time and end_time : Validation issue

// Variables

const minutesToAdjust = 10;

const millisecondsPerMinute = 60000;

const FiveminutesToAdjust = 5;



// Current time

const originalDate = new Date();

var originalMinute = String(originalDate.getMinutes()).padStart(2, "0");

var originalHour = String(originalDate.getHours()).padStart(2, "0");





// Add 10mn by default when open form

const addtimeDate = new Date(originalDate.valueOf() + (minutesToAdjust * millisecondsPerMinute));

var addMinute = String(addtimeDate.getMinutes()).padStart(2, "0");

var addHour = String(addtimeDate.getHours()).padStart(2, "0");



// Read start_time value (if manually modified)

function readStartTime() 

{

var new_start_value = document.getElementById('start_time').value;

var newStartHour = String(new_start_value).substr(0, 2);

var newStartMinute= String(new_start_value).substr(3, 4);



// pick end time string & convert to time



var updateStartTime = new Date();

new_hours =updateStartTime.setHours(newStartHour);

new_minutes = updateStartTime.setMinutes(newStartMinute);



// test Add 5mn

var newEndDate = new Date(updateStartTime.valueOf() + (FiveminutesToAdjust * millisecondsPerMinute));

var addMinuteFive = String( newEndDate.getMinutes()).padStart(2, "0");

var addHourFive = String( newEndDate.getHours()).padStart(2, "0");

var newEndDate = (addHourFive + ":" + addMinuteFive);

document.getElementById("end_time").value = newEndDate;





// if its not edition

var element = document.getElementById('title');

if (element != null && element.value == '') {

document.getElementById("title").value = ("Fermeture");

document.getElementById("start_time").value = (originalHour + ":" + originalMinute);

document.getElementById("end_time").value = (addHour + ":" + addMinute);

}



var 12hour = document.getElementById('view12Hour');

if(12hour.checked) {

    check12hTime(document.getElementById('start_time'));

    check12hTime(document.getElementById('end_time'));

} else {

    checkTime(document.getElementById('start_time'));

    checkTime(document.getElementById('end_time'));

}



// debug

//console.log(new_start_value);

//console.log(newEndDate);

//console.log(addHourFive + ":" + addMinuteFive);

}