Live Broadcasts Do Not Have A Time Zone
Live broadcasts don't have time zone associated with them at all. (Since they are live, they happen at the same time for everyone no matter where they are located.)
There are two points in the process when timezones come into play:
When scheduling a broadcast in Ecamm Live, it will schedule based on your Mac's time zone. (Your Mac's time zone is indicated in System Preferences -> Date/Time.)
When Facebook or YouTube shows your announcement post to visitors, it does its best to show them the go-live time in their own local time zone. (For example, where it says "TODAY AT 5:00 PM", another visitor in a different time zone will see "TODAY AT 7:00PM".)
If you mention the broadcast's time in your post description text, its best to mention the timezone if mentioning the time. For example "Join me live at 4:00 EST!"