Up until now, Google always reported your AdSense earnings based on Pacific Standard Time (PST) but with this change you can now set the reporting to be based on your local time zone. I changed mine to EDT.
To update this yourself, go to the Account information section on your Account settings page, a field now exists for time zone information where you can select your preferred local time setting.
Google notes that when you "update your time zone preferences, you can expect to see some anomalies in your local earnings data for that day."
Forum discussion at WebmasterWorld.