The FAQ says

When?
...
All times will be 12AM GMT.

However, the countdown timer for the jam start is going to 11 AM GMT (11 hours later)

Maybe daylight saving shenanigans going on somewhere? I don't keep track of that stuff. But it is set to 12am, and the timer lines up for me. So I'm not sure.

If you hover over the time, you see the time in UTC/GMT. According to that, it seems to be 11AM UTC currently (living in GMT+2).