The variable only stores the UTC time. The offset is something the runtime adds.
Or do you want to store the local time as utc time? If so you can use a combination of parseDateTimeUTC and formatDateTime like this:
this will convert the time to a string (including offset) and stores it as UTC time.
Try not to confuse GMT offset with locale. Locale is much safer to use (takes care of daylight savings automatically) and is fairly easy to convert from UTC to locale date and back