To do this in PHP you would create an acf/save_post filter, get the two fields, calculate the difference and then update the 3rd field.
add_action('acf/save_post', 'calc_time', 20);
function calc_time($post_id) {
$start = get_field('start_time', $post_id);
$end = get_field('end_time', $post_id);
// calculate difference
// see https://stackoverflow.com/questions/10557076/adding-two-time-values-of-similar-formats-using-php
update_field('total_time', $calculated_time, $post_id);
}
Doing this with JS would be much more difficult and I would not be able to provide any code without spending a lot of time doing so because there are no examples that I know of. YOu would need to add custom JS https://www.advancedcustomfields.com/resources/adding-custom-javascript-fields/ and add onChange actions to the date fields. The you would need to use the ACF JS API https://www.advancedcustomfields.com/resources/javascript-api/ to get the values of the fields, then you’d need to calulate this difference using JS https://stackoverflow.com/questions/12193342/calculate-time-difference-in-hhmm-format and finally use the ACF API to set the value of the 3rd field.
Sorry I can’t help you more with the code, but like I said, I don’t have any examples and building them would require quite an effort.
