Markup: templates/output/04-elements/forms/input/datetime.html
<div class="form-group">
<label for="demoformid-idt">Birthday (date and time)</label>
<input class="" type="datetime" name="" id="demoformid-idt" value="1970-01-01T00:00:00Z"/>
</div>
<div class="form-group has-error">
<label for="demoformid-idt-error">Birthday (date and time)</label>
<input class="" type="datetime" name="" id="demoformid-idt-error" value="1970-01-01T00:00:00Z"/>
<div class="form-group__error" role="alert">This field is required</div>
</div>
Source:
assets/sass/04-elements/_elements.forms.scss, line 219