My problem is that the admin define slots as NUMBER and not as DATE
So if we use the timezone of the user that’s will create a wrong date because it will change the hour of “current date/time” of the user to the slot hour (number define by the admin) but in a wrong timezone 
My goal is to keep a simple admin by define slots as number and just have a static timezone for all my website not taking in account the timezone of the user, it’s not useful and creating errors.
How can i do that ? I read a post that seem to arrive to convert to Date format with changing the timezone but it’s seem to be very hard to do : Get the GMT offset for a Timezone
Have you an idea to just disable the timezone of all visitors and have a static timezone ?