First at all, I'm not develloper

I wrote little JS script (it load with Jumi when user open Create Event frontend page) to complete (prefill) start_time and end_time with this values :
start_time : now
end_time : now + 10mn
This part is ok
The second part of script it's if user change start_time, and click on +5mn button, the function read start_time and recalcul end_time to add +5mn.
But if I specify start_time value posterior at end_time, and click on +5mn, the calcul is ok, end_time is update with new start_time value with +5mn, but the end_time field is red (validation issue ; date/time invalid)
The behavior seems to be the validation doesn't recheck start_time again and probaly see end_time is anterior at start_time..
How I can force revalidation/reload of fields with javascript...?
Thanks
Regards
Philippe
The script :
// 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);
}
// debug
//console.log(new_start_value);
//console.log(newEndDate);
//console.log(addHourFive + ":" + addMinuteFive);
}