Time synchronization is very important for networked computer systems. In addition to correct time with respect to a global standard (UTC/GMT), we also desire to have a more friendly local time display for our users. This would be easy to do if local time was always a fixed offset from central time, but alas – we must accurately reflect local daylight saving time rules as well.
Daylight saving time start and end dates vary from region to region. In the United states, recent laws have extended DST by an additional four weeks (this change began in 2007). To look-up the day that daylight saving time begins and ends, I recommend the Wikipedia page Daylight saving time in the United States. For 2013, DST in the US begins on March 10th (the second Sunday of March).
For computers to automatically show the changed local time, we require a complex database of all localized DST rules. For most Linux and Unix systems, this is accomplished with the tz database (AKA zoneinfo). A copyright dispute in 2011 led to the ICANN taking official responsibility for the maintenance of these time-zone database files which are now available for download from the IANA Time Zone Database site.
All this trouble just to see computer time displayed with automatic adjustments for your local daylight saving time rules! Many thanks to all the people who put their time and effort into maintaining and distributing the time zone database :-).