Oh, I thought that you meant that it was there and that you just wanted to display the value on the listing.
Adding the field in the form in not complex either.
You can just add the code
<tr>
<td class="key">
<label for="data[product][product_sale_start]">
<?php echo JText::_( 'PRODUCT_SALE_START' ); ?>
</label>
</td>
<td>
<?php echo JHTML::_('calendar', hikashop_getDate((@$this->element->product_sale_start?@$this->element->product_sale_start:''),'%Y-%m-%d %H:%M'), 'data[product][product_sale_start]','product_sale_start','%Y-%m-%d %H:%M','size="20"'); ?>
</td>
</tr>
<tr>
<td class="key">
<label for="data[product][product_sale_end]">
<?php echo JText::_( 'PRODUCT_SALE_END' ); ?>
</label>
</td>
<td>
<?php echo JHTML::_('calendar', hikashop_getDate((@$this->element->product_sale_end?@$this->element->product_sale_end:''),'%Y-%m-%d %H:%M'), 'data[product][product_sale_end]','product_sale_end','%Y-%m-%d %H:%M','size="20"'); ?>
</td>
</tr>
in the file infovariant.
However, the values won't be used anywhere. It would require to modify core files of HikaShop to handle the check of these values (in the quantity file of the product view, in the file administrator/components/com_hikashop/classes/cart.php )