[рони]

Владимѣръ,

<script>
let moonBegin = 1613032380000;
let moonMonth = 29.53059 * 24 * 60 * 60 * 1000;
let now = Date.now()
let moonDay = ((now - moonBegin) %  moonMonth)/(moonMonth/30)|0;
document.write(`Сейчас ${moonDay} лунный день`)
    </script>