If user is able to type date in text, do show the date format somewhere, i.e.:
Start Date: <input type="text" name="start_date" placeholder="yyyy-MM-dd">Start Date <small>yyy-MM-dd</small>: <input type="text" name="start_date">Start Date: <input type="text" name="start_date" alt="yyy-MM-dd">- combination of (all) above