phpBB doesn't implement client-based daylight savings, because it's used globally, and daylight savings is HARD to implement correctly. That's because some places do daylight savings, some don't. Some go forward from the standard time, others go back during a different part of the year. Some change by and hour, others change by only a half hour.
Now, the time is based off the server time, so if the server is set to use daylight savings locally, then the official forum time will be based on that.
But, the last news I saw, was that phpBB wouldn't be implementing a client setting for daylight savings.