2013-07-29 12:16 pm EST
Thanks for the question. It really depends on your software (or how your ecommerce site is programmed). The program generally has a set date for your deadline of your sale/discount and if there are not time specifics, then it will end midnight of the 30th, since July 30 is your end day. It will include the day.
Remember to think about your time zones as well. If you're taking sales from multiple time zones, then some areas will of course see the discount expire a few hours differently from the other time zones. For example, if your server is on the East coast of the US, and your expiry date is the 30th, then it will occur 3 hours earlier for West Coast clients. So you might need to make your sales/discounts/coupons regional if you want to apply the expiry date across your customer base evenly.
Here are some articles we have written that demonstrate setting a discount/sale/coupon in OpenCart, and one for creating coupons in Prestashop. Note that the OpenCart example does not specify a time, whereas the PrestaShop version has the option to set the expiration to the minute.
If you're just declaring a sale since the program you're using doesn't have a way to make the sale/discount expire, then it's really up to you, but if you declare that the sale is done on the 30th of July, then it should generally include that day.
I hope that helps to clarify the issue for you! Please let us know if you require any further assistance.