The change is as follows. If you are in the Eastern Timezone and you follow DST then your timezone is now -4 GMT. More info and more timezone are here:
How to Change Guild Settings
The Guild Timezone is set in the Guild Admin area using the link that says "Guild Settings" on the left of the main Admin page. On the page that link leads you to is an edit form for the timezone.
How to change personal timezeon
Your profile timezone is on the page linked at the top right of the page when you are logged in. About 2/3 of the way down the page are the timzeone settings.
The need to make this adjustment will be removed in the 7.0 release.
I have a question
How long it will be in this region and when to change the clocks again
With changes in the 7.0 release you won't have to change your timezone again. You only need to enter your Standard timezone and then whether you follow DST. If you are in Europe then most likely you just need enter your GMT offset and you are done.
Is it possible to have your timezone affect the listings in the Calendar parts of a guild site? (That is, instead of seeing calendar entries in the timezone that the original poster used and having to convert manually, can I see them in my own timezone?)
As of now we do not have a way for it to automatically convert calendar event timestamps into the users timezone. This is a great idea though! Pop over to our uservoice and drop a feature request there. I'll make a note of it in our system, but uservoice is open to all users and they can vote and comment on your suggestions. This just helps push it along for development!