All iOS developers will be familiar with the reports produced through iTunes Connect to track activity in the App Store relating to your applications.
Some of these reports are daily, which leads to the slightly vexing question of how a day is defined. It sounds like a simple enough question, which should have a definitive answer. And it does, but it's not clear that it is the right answer.
The Apple documentation states that a day is midnight to midnight PST (Pacific Standard Time). The problem is that the data just does not seem to match this.
Most of the time it is hard to verify, but when an app first goes into the App Store it can be possible to correlate some specific events with when they show up on the reports. This happened when the Grimes App went into the App Store last week, and the simple definition does not seem to work.
I have seen websites trying to explain these discrepancies as due to credit card processing times, but this doesn't make sense for free apps.
There is a tantalizing hint in the documentation when it refers to PST as the ”last” timezone. This led me to wonder whether these reports actually cover local midnight to midnight for each time zone. In other words do they collate all of the activity that happened on a given day according to the local time? So if you downloaded an app on Friday according to your local time, it would appear on the Friday report, regardless of which day it was in the PST timezone.
For example, consider an event that occurred at 8am Friday AEST (UTC +10) = 2pm Thursday PST (UTC -8). Using the simple PST definition this would be included on the Thursday report. Using my convoluted local time definition it would be included on the Friday report. This is consistent with what I observed last week.
There are a couple of flaws to this theory. Firstly, although it has some elegance it seems overly complex (and is contrary to the documentation). Secondly, PST would not be the ”last” timezone in this regime. Timezones to the west of the PST zone and east of the International Date Line would need to make it to midnight before a full days report could be compiled. These timezones include Alaska and Hawaii, and various other Pacific islands.
But perhaps this is not such an issue. The US states are covered by the US App Store, and I am not aware of any App Stores specific to the non-US Pacific islands east of the date line. So if the US App Store is reported based on PST, PST would indeed be the ”last” timezone. This would diminish the elegance of the solution as Hawaii and Alaska would be exceptions to the ”report it on the local day it happened” rule (eg a purchase made at 11pm Thursday in Hawaii (UTC -10) would be included the report for Friday)
Of course this is largely irrelevant and really only a matter of intellectual curiosity. Unless Apple were to issue some sort of clarification (highly unlikely) it is likely to remain a mystery.