As a european user, i think you should use the IP address of the user to determin the time zone and have an opening time for every time zone!
For the love of all that's holy, never ever ever use the user's IP address to determine their locale, language preferences or anything similar. It works for the majority of the users and is an absolute nightmare for the rest. Can't even begin to count the times a site has thought I'm from some random country and then switched languages, leaving me to look for the language picker that's hidden somewhere in the page footer. Don't do that.