Hi,
There is already a start and end date for each product.
The manufacturer id is directly in the product table so you could really easily change the start/end date of all the products of a manufacturer with one MySQL query run in your phpmyadmin. Something like that:
UPDATE jos_hikashop_product SET product_sale_start=X, product_sale_end=Y WHERE product_manufacturer_id=Z;
where X and Y are dates in the unix timestamp format
www.google.com/search?sourceid=chrome&ie...&q=date+to+timestamp
and Z the id of the manufacturer.
