Here's a rundown on how TimeZones are used in DNN.  I thought I should include in this thread:

The xml file (TimeZone.xml) is offset based as most files are (including the Windows Registry).  So, if you calculate the offset from +0 (zero) or :

 <timezone name="(UTC  00:00) Dublin, Edinburgh, Lisbon, London" key="0" />

Example, right now, it's 11:28 PM in Dublin; my timezone is Eastern and the Time is 6:28 PM...so I am -5 hours behind (including the current DST)... so -5 * 60 = -300....and my entry in the TZ file would be:

 <timezone name="(UTC -05:00) Eastern Time (US &amp; Canada)" key="-300" />

This is how all timezones are calculated.  The problem w/the DNN TZ Table is it has no way to update it (no User Interface)...so you need to update it directly (manually).  Also, the entries in it do not include when DST goes into effect (start/end date/time and offset)...I have been trying to get the core team to address this, as it is not just an issue for the Events module...it effects any module that required the correct Date/Time for the Site!  There are other issues...but that's another entry....Hope this helps.