Countdown a 1 hour discount on all sites, but only once for each user. Get current time and endtime is currenttime + 1 hour

i think i need to create a session-variable that safes the current time of the user on first pageview. I hand over the timestamp to the site with a meta-tag and with js i will count the current time down to currenttime + 60 Minutes. After 60 Minutes the Countdown should disappear for the user.