`update_post_meta` not working anymore

Question

Originally my goal was to compress my code, now after I altered it, it is not working anymore. I can't figure out why. I simplified it a bit, now this code should save one date entered in the backend in a custom meta box of a custom post type but apparently the update_post_meta() function is not working anymore. E.g. if I select a date and save the post, the change isn't saving.

add_action ('admin_init', 'cpt_event_meta_fields');
add_action ('save_post', 'cpt_event_save_data');
    function cpt_event_meta_fields() {
        add_meta_box("event-meta-1", "Datum (von)", "cpt_event_callback", "Termin", "normal", "high");
    }
    function cpt_event_callback( $post ) {
        $dateStart = get_post_meta( get_the_ID(), 'date-start', true );
        echo $dateStart;
        echo '<input type="date" name="date-start" value="' . $dateStart . '" />';
    }
    function cpt_event_save_data() {
        global $post;
        update_post_meta(get_the_ID(), "date-start", $dateStart);
    }

After reviewing the answer below is myupdated code. My main problem was the use of global $postoutside of the Loop. I used global $_POST instead. Also I now changed the input from date to text and sanitized the textfields using sanitize_text_field()

add_action ('admin_init', 'cpt_event_meta_fields');
add_action ('save_post', 'cpt_event_save_data');
function cpt_event_meta_fields() {
    add_meta_box("event-meta-1", "Datum (von)", "cpt_event_callback", "Termin", "normal", "high");
    add_meta_box("event-meta-2", "Datum (bis)", "cpt_event_2_callback", "Termin", "normal", "high");
}
function cpt_event_callback( $post ) {
    $dateStart = get_post_meta( get_the_ID(), 'date-start', true );
    echo '<input type="text" name="date-start" value="' . $dateStart . '" />';
}
function cpt_event_2_callback( $post ) {
    $dateEnd = get_post_meta( get_the_ID(), 'date-end', true );
    echo '<input type="text" name="date-end" value="' . $dateEnd . '" />';
}
function cpt_event_save_data() {
    global $_POST;
    global $dateStart;
    $dateStart = sanitize_text_field($_POST['date-start']);
    $dateEnd = sanitize_text_field($_POST['date-end']);
    update_post_meta(get_the_ID(), "date-start", $dateStart);
    update_post_meta(get_the_ID(), "date-end", $dateEnd);
}

Answer 1

get_the_ID() is a "Loop" function. It won't outside the Loop, and there is not Loop in your code. Essentially, you are passing a bad ID to the function causing it to fail. If you had debugging enabled you might see feedback to that effect.

You are also using a variable-- $dateStart-- that is not set in the context of the function.

You are not properly using the save_post hook, however. WordPress will pass the post ID through to your callback if you ask for it.

Finally, what you want is global $_POST; which is the PHP form data, and not global $post; with is a WordPress variable that is a primary component of Loops.

function cpt_event_save_data($pid) {
    global $_POST;
    $dateStart = $_POST['date-start']; 
    // sanitize $datestart, maybe with PHP's checkdate()
    // I don't know what the format of your string is though
    update_post_meta($pid, "date-start", $dateStart);
}

Untested, but that should be pretty close.