Getting a null value of input fields on mobile, but getting value on desktop

I am setting some values of input fields in local storage through JavaScript, it is working fine on desktop but when i check it on mobile i am getting null values.

Here is my code:

function reserve() {
  let date = document.getElementById("start").value;
  alert(date)
}

<input type="date" class="date" id="start" placeholder="Move Date">
<a href="/request"><input type="button" name="" value="Reserve Now" onclick="reserve()"></a>

Now i am getting null on mobile but get a value in alert on desktop.