Javascript – How to calculate age in YYYY years MM months DD days

I have been trying to make an age calculator which is a challenge from front end mentor. I have figured out the logic to finding the years and months using the Date() method in JavaScript. But my days age is not accurate, i cant seem to figure out the logic behind how I can show the number of days old someone is according to different months, as some months have 30 some 31 and February has 28.
here is my code below.

const days = document.getElementById("days"); // the day input box
const months = document.getElementById("months"); // the month input box
const years = document.getElementById("years"); // the year input box

function calculateYears() {
  var dayValue = document.getElementById("day").value; // day value
  var monthValue = document.getElementById("month").value; //month value
  var yearValue = document.getElementById("year").value; //year value

  var birthday = new Date(yearValue, monthValue - 1, dayValue); // birthday in milliseconds

  var today = Date.now(); // todays in milliseconds
  var diff = today - birthday; //age in milliseocnds

  var currentAgeYears = Math.floor(diff / 31556952000); // age in milliseconds/ by milliseconds in a year
  var ageMonths = Math.floor(diff / 2629746000); // age in milisecond / by milliseconds in a month
  var currentAgeMonths = ageMonths % 12; // remainder  total age in months / 12 to get age in current year's month
  var ageDays = diff / 86400000; // age in milisecond / by milliseconds in a day
  
  
  
  currentAgeDays = Math.floor(ageDays % 30); // this is where i need help what logic can i use for days 

  years.innerHTML = currentAgeYears + " "; 
  months.innerHTML = currentAgeMonths + " ";
  days.innerHTML = currentAgeDays + " ";
}

i need help in this line of code
`var ageDays = diff / 86400000; // age in milisecond / by milliseconds in a day

currentAgeDays = Math.floor(ageDays % 30); // this is where i need help what logic can i use for days`

i am going to attach a image of my output. as the day of testing this it is August 24 2023. the output should be 1 year 0 months and 0 days yet the days don’t match.

Any help would be appreciated enter image description here