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