I don't know about the specified times and dates, but when making the jam itch asks me for info in my timezone. The countdown on the jam page is set for your timezone though, so if you're not sure, just use the countdown to find out the time in your own timezone.
As for the late submission period, it is based on my timezone, but if you have the end of the jam's hour, just add 72 to it to find out when the late submissions end.