Hello,
The issue is regarding the configuration of the timezone of your server.
When the timestamp is not set, the value in the database is "0".
That value is sent to the function "hikashop_getTime" ; which convert a date into a timestamp.
But when you give it a timestamp, you should get the same value in return. In your case it gave you the timestamp plus one hour.
It is also why when you re-save a product, the dates increase.
First time : "1970-01-01 01:00"
Second time : "1970-01-01 02:00"
Etc...
Please note that it is also visible in HikaShop and for all date fields that you can edit.
Even if I add a patch into HikaMarket to avoid the save of the date when the value is "0" ; you would still have your timezone configuration problem and the increase of the date each time you save the product.
So you can edit the HikaMarket product class and replace
$product->prices[$k]->price_start_date = isset($value['price_start_date']) ? hikamarket::getTime($value['price_start_date']) : '';
$product->prices[$k]->price_end_date = isset($value['price_end_date']) ? hikamarket::getTime($value['price_end_date']) : '';
By
$product->prices[$k]->price_start_date = !empty($value['price_start_date']) ? hikamarket::getTime($value['price_start_date']) : '';
$product->prices[$k]->price_end_date = !empty($value['price_end_date']) ? hikamarket::getTime($value['price_end_date']) : '';
But it would just hide the problem in your case.
Best regards,