JSFiddle - React, Tailwind, and code Playground

HTML

<input type="date"  id="checkin" name="checkin" placeholder="dd-mm-yyyy" value="<?php if ( isset( $checkin ) && ! isset( $email_sent ) ) echo esc_attr( $checkin ); ?>" />

CSS

input[type='date']{
	width:20%;
    height: 38px;
	padding:0 2%;
    margin:0 0 20px 0;
    border-radius: 0;
    outline:none; 
   -webkit-appearance: none;
	box-sizing:border-box;
	-moz-box-sizing:border-box;
	-webkit-box-sizing:border-box;
	border:1px solid rgba(0,0,0,0.08);
    font-size:1em;
    line-height:1.4em;
    color:#808080;	
}

JavaScript

$date = "12-31-2019"; // m-d-Y
$converted_date = date("d-m-Y", strtotime($date) );