When creating a countdown command to EST time with $(countdown August 27 2016 10:00:00 PM EST), it is one hour ahead (or behind, not sure), while the command $(countdown August 27 2016 10:00:00 PM America/New_York) displays it correctly.
With EST as timezone: Nightbot: I will be streaming in: 6 hours 40 minutes 3 seconds
With New York as timezone: Nightbot: I will be streaming in: 5 hours 40 minutes 3 seconds
It will always be best to use the area in the countdown variable, as it won’t be subject to any such changes and will change along with the time if it goes from EDT or EST or vice versa. If you pick the timezone it will be constant, however.