Entering text into input does not change value. How to fix?

Question

JavaScript

There is an input field: I'm trying to get the entered value using: (All this, of course, is inside a function that is a handler for changing the input)
<input id="datetime_from" type="text">

let input = document.getElementById("datetime_from");
let result = normalizer(input.value);

But value is always empty, even if something is entered and I then try to get the value. Those. The error isn't that the handler doesn't fire.

UPD.
Just in case, I give the normalizer code

let normalizer = function (date) {
        let res = /(\d\d).(\d\d).(\d\d\d\d)/.exec(date);
        if (!res)
            return '';
        let normalized = `${res[3]}-${res[2]}-${res[1]} 00:00:00+03:00`;
        return normalized;
};

UPD.2
All this is inside the django admin with the suit theme.
UPD.3
Even if I just open the page, enter text in the field, and then write in the browser console

console.log(document.getElementById("datetime_from").value)

still empty.
(Which actually illustrates the screenshot).
Both firebug and built-in dev tools give:
<input id="datetime_from" type="text">

Answer 1

Maybe replace let with var?
Then maybe see what happens without the normalizer()

Answer 2

You may have 2 or more #datetime-from there (i.e. with one id)

Answer 3

Can you add an event handler?
Not much, maybe it will help?
window.addEventListner('onchange ', handler, somethingElse);