This works just fine. Just get the name fields with inspect element. Add this code in the functions.php file.
add_action('acf/validate_save_post', 'my_acf_validate_save_post', 10, 0);
/**
* @throws Exception
*/
function my_acf_validate_save_post() {
$start = $_POST['acf']['field_61a7519a57d99'];
$end = $_POST['acf']['field_61a751d957d9a'];
// check custom $_POST data
if ($start > $end) {
acf_add_validation_error('acf[field_61a751d957d9a]', 'End date should be greater than or equal to start date.');
}
}
When you open inspect element the input field should look like this:
<div class="acf-date-picker acf-input-wrap" data-date_format="dd. MM yy" data-first_day="1">
<input type="hidden" id="acf-field_61a751d957d9a" name="acf[field_61a751d957d9a]" value="20211201">
<input type="text" class="input hasDatepicker" value="16. January 2022" id="dp1638477022818">
</div>
More information you can find here:
https://www.advancedcustomfields.com/resources/acf-validate_save_post/