Calculate days and weeks from date in javascript

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var lastMestrualDate = document.getElementById('LastMestrualDate').value; const oneDay = 24 * 60 * 60 * 1000; // hours*minutes*seconds*milliseconds const firstDate = new Date(lastMestrualDate); const secondDate = new Date(); const diffDays = Math.round(Math.abs((firstDate - secondDate) / oneDay)); var weeks = diffDays / 7; var days = diffDays % 7; document.getElementById('EGA').value = parseInt(weeks)+' Weeks' +' '+ days +' Days';