I’ve just made this silly and simple countdown to have in my phone homescreen and I think maybe you like it and want to use.
Here the instructions for Android Chrome,
Visit this url in your phone, with your arrived date as fecha param
Once the page has loaded, click in the menu and then in Add to Home screen
For iPhone, I thing the procedure is tapping in Share button, then in the dialog choose Add to Home Screen though I can’t test it.