So yeah, the current window wouldn't work for me, but that's fine. Everything doesn't have to be for everyone. We all live in our bubbles anyway; creating artificial rules could actually be ways of creating new, unexpected interactions.
This being said – if you were to adjust the rules to accommodate more people, I don't think it should be "open from 7:39 to 10:39 in whatever your local time zone is", because that feels like it would just destroy the whole idea – that everyone is there at the _same_ time. Also, it would still exclude people who work evenings.
An alternative solution would be to have multiple windows. For example, if you have one starting at 7:39 PM EST and another one at 7:39 AM EST, there would be more chances that there is some time during the day for people around the globe to check in. Depending, of course, on many things: time zones, sleep habits, work schedule, ability to briefly slack off during work, etc. It would remain true to the idea while opening up for some more people. Just a thought.
I also think each window could be smaller, maybe like just one hour?