Expiry time is the start of the day entered (if chosen). For example, an expiry date of 07/09/2010 will expire at 12:01AM 07/09/2010 and will not be valid through the day entered.

The time zone used is that set for your shop on your Preferences > General Settings page (if using Classic Shopify admin).

If using Shopify 2, you can find out which time zone your store is set in by going to Settings > General in your admin panel, and scrolling down to "standard formats."