Can you try to run this code in your local machine?
Make a file name 55788817.php and paste this code and run.
<?php
if (isset($_POST) && !empty($_POST)) {
$json = null;
if(isset($_POST['json'])) $json = json_decode($_POST['json']);
var_dump($_POST); //Result: array(1) { ["json"]=> string(29) "{"ts":1555915560755,"tz":5.5}" }
exit;
}
?>
<!DOCTYPE html>
<html>
<head>
<title></title>
</head>
<body>
<button type="button" id="registerButton">Click Me!</button>
<script>
var now = new Date();
var pkg = JSON.stringify({
ts: now.getTime(),
tz: now.getTimezoneOffset() / -60
})
var form = new FormData();
form.append('json', pkg);
console.log(form.has('json')) // true
console.log(form.values().next()) // return and obj contain JSON string
fetch('./55788817.php', {
method: 'POST',
body: form
});
</script>
</body>
</html>