how to handle subtraction in thymeleaf - thymeleaf

Question

I have registration form, where a user can enter her
name,email,dateofBirth,mobile Number. I want to calculate the users
age. If the user is 21 old he is able to register with us unless he cannot able to register. I have already created custom validation, and I pass the validation in my model class. But I am not able to calculate the age and use the logic for 21 year. I created dynamic form.
SignUp.html
<div class="content-area-container page-ath-wrap" th:object="${advertisement}">
<div class="page-ath-content container-fluid">
<div class="page-ath-form">
<div class="page-ath-heading">
<!-- <h3 th:text="*{reward_amount}">Congrats! You just earned $5 off your next beer.</h3>-->
<h3 th:text="*{form_title}"></h3>
<!-- <p>Enter your info to claim the reward!</p>-->
<p th:text="*{form_description}"></p>
</div>
<div class="singup-form">
<form id="form_signup">
<input id="id" th:field="*{id}" type="hidden"/>
<div th:each="item,iterStat : *{form}" >
<!-- <label th:label="${str.getLabel()}" th:type="${str.getType()}" th:text="${str.value}" ></label>-->
<div class="form-group">
<label th:for="${item.name}" th:text="${item.label}"></label>
<div th:if="${item.validation == 'dob'}" >
<input class="form-control" th:id="${item.name}" th:name="${item.name}" th:placeholder="${item.placeholder}" th:required="${item.required}"
th:type="${item.type}" th:attr="max=${#dates.format(#dates.createNow(), 'MM-dd-yyyy')}"/>
</div>
<div th:unless="${item.validation == 'dob'}" >
<input class="form-control" th:id="${item.name}" th:name="${item.name}" th:placeholder="${item.placeholder}" th:required="${item.required}"
th:type="${item.type}"/>
</div>
</div>
</div>
Advertisement.kt
class Advertisement(val media: ArrayList<MEDIA>? = null)
class Form(
val type:String="",
val required:Boolean,
val label:String="",
val placeholder:String="",
val name:String="",
val value:String="",
val validation:String="",
val min:Int,
)

Answer 1

As other answer suggested you can do it with javascript.
You can make use of this jQuery validation API
And add a custom validation method like so:
jQuery.validator.addMethod("olderThan21", function(value, element) {
return //age calculation logic if true-> validation pass
//if false->validation error }
, "You must be 21+ to subscribe");
In case that the validation isn't passed satisfactory this will prevent the form from submitting.