The goal of the chat event being advertised is to call attention to not only the event itself, but to chat itself. If someone clicks on the event and they are there 15 minutes before the official start, I don't see this as a problem. They still got a "yo, something's up, come join us!" message, and they did come join you.
If the banner doesn't go up until right when the event starts, we aren't really advertising the event to the community so they can join for the start. A user might be around, but not actively reloading the site during the event. They can see the message ahead of the event and decide to attend.
Putting the UTC time in the system message isn't a good option because (although I personally don't understand the confusion) people would be confused about when the event was. Even if you show the time in the user's local time, it still doesn't have that sense of "now" urgency.
A countdown in the system message would either by incorrect or distracting. Do we count down in intervals of 10 minutes? 5 minutes? But then it still isn't correct in the interim. The only way for it to be accurate would be to count down by the second, and this would be a distraction to the user.
Saying "right now" has a sort of "instant gratification" feeling to it. It can help grab attention and get people interested in your event. That's exactly what you want! (: