The guest timezone is the only control you have over the timezone other users will browse the forum with. The initial admin user (you!) will also use this guest timezone by default.
For people registering, we detect their timezone automatically to some degree of accuracy so for most users registering it should be correct without intervention, regardless of what the guest timezone is.
If your timezone is wrong on your own account (which it will be by default) then you can change it in the Account menu > Preferences on the front end.
Users can change their own timezone that way to anything they want, any time in the same place.