Bootstrap 4 datapicker.js not included

You can use this and then you can add just a class form from bootstrap. ^{(does not matter which version)}

<div class="form-group">
 <label >Begin voorverkoop periode</label>
 <input type="date" name="bday" max="3000-12-31" 
        min="1000-01-01" class="form-control">
</div>
<div class="form-group">
 <label >Einde voorverkoop periode</label>
 <input type="date" name="bday" min="1000-01-01"
        max="3000-12-31" class="form-control">
</div>

Maybe you want to try this: https://bootstrap-datepicker.readthedocs.org/en/latest/index.html

It's a flexible datepicker widget in the Bootstrap style.