I have created as Yahoo Calender link and it doesn't seem to be respecting the timezone for the end date:
https://calendar.yahoo.com/?v=60&TITLE=Chapter%20Luncheon%20-%20CO%20-&ST=20151022T170000Z&ET=20151022T190000Z&URL=tma.utdev.com&in_loc=Maggiano%27s%20Little%20Italy%20500%2016th%20St%20Ste%20150%20Pavilions%20Mall%2C%20Denver%2C%20CO%2080202%2C%20United%20States&desc=
This displays 1:00PM to 7:00PM. I am in the EST Timezone which is GMT-4:00, so 17:00:00 (aka 5:00) - 4 hours = 1:00 PM. But for the end time, 19:00:00 (7:00), its not converting and its leaving it at 7:00
For google it does work:
https://www.google.com/calendar/render?action=TEMPLATE&text=Chapter+Luncheon+-+CO+-&dates=20151022T170000Z/20151022T190000Z&sprop=website:tma.utdev.com&location=Maggiano%27s+Little+Italy+500+16th+St+Ste+150+Pavilions+Mall,+Denver,+CO+80202,+United+States&details&website=http://tma.utdev.com/event/chapter-luncheon-co&pli=1&sf=true&output=xml#eventpage_6
This displays the date 1:00PM to 3:00PM
From what I can tell, Yahoo Calendar doesn't support an ET parameter. (If it does, it's undocumented and buggy). I'd therefore recommend using the DUR parameter instead, to specify the duration:
https://calendar.yahoo.com/?v=60&TITLE=Chapter%20Luncheon%20-%20CO%20-&ST=20151022T170000Z&DUR=0200&URL=tma.utdev.com&in_loc=Maggiano%27s%20Little%20Italy%20500%2016th%20St%20Ste%20150%20Pavilions%20Mall%2C%20Denver%2C%20CO%2080202%2C%20United%20States&desc=
See http://chris.photobooks.com/tests/calendar/Notes.html or http://taskboy.com/blog/Creating_events_for_Yahoo_and_Google_calendars.html for more information on the supported parameters.