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I have seen, many times, people announcing some event on the Internet in terms of EST (Eastern Standard Time) or, slightly less often, PST. In the winter, there's no practical problem with that, but for slightly more than half of the year, this terminology comes with a significant “gotcha”. See, chances are, whoever posted the announcement actually means not “Eastern Standard Time” but “the time in New York City” – or, instead of “Pacific Standard Time,” they mean “The time in Los Angeles.” The problem is that, from March to November, New York City doesn't follow Eastern Standard Time; it follows Eastern Daylight-saving Time! When we change our clocks in the spring and fall, Standard Time doesn't change; rather, we switch between it and Daylight-saving Time. So in the summer, while Eastern Standard Time still exists, nobody follows it (except for the west coast of South America, but they surely don't call it that). Most of Indiana did until 2007, but not anymore. And while Phoenix and Denver are both in Mountain Time Zone, only one of them is actually on Mountain Standard Time in the summer.
There are a few solutions. First and best, you could say EDT when Daylight-saving Time is in effect, and EST otherwise. Or, especially when referring to geographic areas and not actual times, say ETZ. The laziest way to specify what time zone you're talking about correctly, especially if you don't know whether DST is (or will be) in effect at the specified time, is to simply say ET if you mean the time in New York, and PT if you mean the time in Los Angeles. (And of course CT for Chicago, and MT for Denver…) I believe that's what CNN does.
The bottom line is this: If you are based in New York and announce some kind of deadline as “11:59PM EST on June 30”, and someone submits something when your clock says 12:45 AM July 1st, that submission is not late, because on Eastern Standard Time, it's still only 11:45 PM. Sure, you could argue about it, but it's just better to say what you mean in the first place and avoid any confusion entirely.