Question

What is preferred isset($_POST['start_time']) or @$_POST['start_time']

Answer 1

Neither.

isset is "this exists and has a value", and that’s not always what you want.

Consider array_key_exists instead:

if(array_key_exists('foo', $_POST)) {
    // code in here will operate if the key exists
    // and can do whatever it needs with the value.
}

Too annoying? Also consider filter_input, with the appropriate INPUT_POST or INPUT_GET flag. Shorter, neater, built-in validation, and no notices about missing keys.

$foo = filter_input(INPUT_POST, 'foo', FILTER_SANITIZE_NUMBER_INT);