PDT stands for Pacific Daylight Time. It is minus seven hours from UTC, or Coordinated Universal Time in Europe, versus EDT, which is minus five hours.
Note that PDT is a daylight saving time/summer time zone. It is generally only used during the summer in the places listed below, during the winter PST is used instead.
PDT is used in the summer in these US states (PST during winter)
California
Idaho - western counties
Benewah, Bonner, Boundary, Clearwater, Kootenai, Latah, Lewis, Nez Perce, Shoshone and north part of Idaho
Nevada
Oregon - except most of Malheur county
Washington
PDT is used during summer in these Canadian provinces (PST during winter)
British Columbia - except for these eastern communities
Cranbrook, Golden, Invermere which is in the MST/MDT zone
Yukon
PDT is used
AS far as eBay goes, are you certain your computer is set in EST? If you are simply trying to change the view in my eBay that shows when the auction ends, you may find it easier to change the preference to showing the time remaining, instead of the time the auction ends. That way, there is no calculating required. Simply go to My account > Preferences > General Preferences > My eBay, and select the time format that shows time remaining. Then instead of it shoing the auction ending at 17:15 on March 30th, it will simply say they are 4 hours, 12 minutes and 10 seconds remaining.
To check the time zone on your computer, left click the time on your screen, select Adjust Date and Time, and then make sure you are in the correct zone.