From : Blog Entry>>Ann’s Blog Entry
There may be times when you need to adjust the time zone for a data set. For example, where you process the data in Coordinated Universal Time UTC (aka Greenwich Mean Time or GMT), but the action itself is in Pacific Standard Time (PST).  To do this, you can create a Time Zone Offset field (or several of these depending on the nature of your case) to display the date/time information for that particular time zone.  Note that you cannot change this information on the document itself, ONLY in a field. [More]