alexmngh,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
</head>
<body>
<form id="form01" th:method="POST" th:action="@{/add-car/new}" th:object="${carEntity}">
<input type="text" id="input01" th:field="*{time}" value="21:45">
<input type="time" id="input02" th:field="*{time}">
...
<button type="submit" class="btn btn-primary">Save</button>
<button type="submit" class="btn btn-primary">Delete</button>
</form>
<script>
document.addEventListener( "DOMContentLoaded" , function() {
document.querySelector('.btn').addEventListener( "click" , function(event) {
event.preventDefault();
document.getElementById("input02").value = document.getElementById("input01").value;
});
});
</script>
</body>
</html>