I recommend checking "Forms" section of "Skeleton" plugin.
To add date type of field, your code should look like:
1. Controller.
$form->getElement("dateselect");
2. Classes
$foo = new AgeRange("dateselect");
$selectField->setLabel($language->text("pluginprefix", "text_key_here"));
$this->addElement( $foo);
3. Views.
<tr class="ow_alt1">
<td class="ow_label">
{label name="dateselect"}
</td>
<td class="ow_value">
{input name="dateselect"}
</td>
<td class="ow_desc ow_small">
</td>
</tr>
Note: all form elements are described on ow_core/form_element.php file.