Refresh Cookie every new day...

Hello ! I got pages that use cookies. The cookie should be alive for 7 days. If the user comes again to my pages, the cookie should be valid for the NEXT 7 days. What I did is to set the cookie on every page, with expire time()+7*24*60*60. This works fine. The problem is, that some users have selected "Ask, before accepting a cookie" - so they are ask every page ! How can I see how old a cookie is to update the cookie only if one or more days have past ? Regards Martin