ily.li/assets/js/fm.js

document.addEventListener('DOMContentLoaded', () => {
    const neverExpireCheckbox = document.getElementById('field-never_expire');
    const expireAfterDaysField = document.getElementById('field-expire_after_days');

    neverExpireCheckbox.addEventListener('change', () => {
        // noinspection RedundantConditionalExpressionJS
        expireAfterDaysField.disabled = neverExpireCheckbox.checked ? true : false;
    });
});