I have two datetime-local fields in odoo template.

<div class="form-group form-field">
    <label class="control-label" for="book_start_time">Start Date and Time</label>
    <div >
    <input type="datetime-local" class="form-control o_website_form_input" name="book_start_time" t-att-value="request.params.get('book_start_time', '')" required=""/>
    </div>
</div>
<div class="form-group form-field">
    <label class="control-label" for="book_end_time">End Date and Time</label>
    <div >
    <input type="datetime-local" class="form-control o_website_form_input" name="book_end_time" t-att-value="request.params.get('book_end_time', '')" required=""/>
    </div>
</div>
How will generate an Warning message when the user inputs an end date less than start date?
Also I need to deactivate all  dates before today's date. Please help