ok, I stole that feature from a newer MyBB version and it's enabled by default now.
It's not actually based on geographical location, but on your local system's time zone. If the client-side time differs from the server-side time by 1 hour, the DST setting will be changed.
It should work as long as your browser has javascript enabled and supports XMLHttpRequest, and you have your time zone set correctly in the UserCP.
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