Policy Activation Time ≠ Policy Run Time, or at least generally speaking it doesn't. It just means when the policy goes from being disabled to enabled. As for when it will run on your clients, it could be shortly after it gets enabled on your JSS, especially if the policy uses Recurring check-in as the trigger, and assuming the Macs are checking in around that time. But of course the run time can be controlled by the Client-Side Limitations tab/settings.
To answer your general question though, the activation time is dictated by wherever your server is located. So for example. if the JSS is located on the East Coast, and you've set the activation time to US 8:00 AM Monday, then when its 8 AM Monday on the East Coast US time, it will activate, even though that may be only 5 AM for any West Coast US clients, or 1 PM in Europe or whatever.
