I'm sorry but I don't think this problem is related to Hikashop so we cannot help you.
Moreover with a description of your problem like this it's quite difficult to help you since you don't even name the calendar plugin you are using.
If you could give a link to the page where you have your custom field of the type "date" we could have a look but I'm afraid that there is a high chance that the problem comes from the calendar library itself which is in joomla and used by HikaShop.
