findMissedBlazingFast.js

const step = 24 * 60 * 60 * 1000;
function findMissed(values) {
    const dates = values.map(x => new Date(x.date).getTime());
    const sum = dates.reduce((acc, x) => acc + x);
    const min = Math.min(...dates);
    return new Date((min * (dates.length + 1) + step * (dates.length + 1) * dates.length / 2) - sum).toISOString().split('T')[0];
}