It says the jam starts on the 11th, ends in 10 days, the 21st, are there times associated with these days and a time for when the theme will be revealed?  Is it just that the theme will appear at 12:00 UTC, 7 hours before the 11th in my own time and then everyone can start?

Start time is when that clock reaches 0. That's the event local time, and even if it's earlier for you, that's when it starts. All event dates are ruled by that clock on the home page. 

Just for reference, in case you or itch.io want to try to fix it, it says 11th to 21st, then it says 10th to 20th at 10PM (my time, which I'll take as correct because the timer matches, ty), then the bottom says 00:00 on the 11th  to 00:00 on the 21st, GMT-3, which is not matched to my time which is GMT-5. My time should be GMT/UTC 03:00 AM and not 00:00.  For reference, I have ad blockers and such disabled on itch.io, so it seems to be a page formatting or internal server code  issue.