As RobertK said, all our stuff is in UTC.
Many XMLTV grabbers can generate a local offset "like 00:00:00 -0400" to make it more readable. A properly written application should take the offset into account when parsing the timestamp so it doesn't matter. Some applications are not written properly, so you need to set a grabber time-offset for the application to work correctly. (and change it with DST).