All events are counted based on the time zone of your Heap account.
For example, let's say your Heap account is set to EST, and a user completes an event at 9:30 PST on May 1st. This event will be counted towards the day after, since the event took place at 12:30am EST on May 2nd.
For more information on the time zone setting, see How do I set my time zone, and what does it affect?