What Xalos said
UTC offset and timezone are different things; the former is part of the latter. Not all timezones observe DST, and when they do, the start and end date varies between timezones.
Not all even change by one hour, there are a few that change by 30 minutes
Now, depending on what you're using it for, UTC offset may be sufficient. However, if it's the actual timezone you want, you'll need to do some sort of geolocation based on IP