Previously, I looked at the iPad 1 (Wi-Fi only) internal clock and found that it is left to drift on its own. Without aid from a cell tower clock, like the GSM models, or periodic sync to a Mac, the iPad 1 clock has no way to set itself to a time standard. Worse, apps are forbidden by Apple to reset the internal clock. Testing of an iPad 2 shows that its clock is no better in this regard.
Back in February, 2011, I found that a Wi-Fi only iPad 1 was not invoking an NTP daemon to check with an external time standard over that Wi-Fi connection. Apple seems to have determined that this is not necessary, even though, without constant calibration, the iPad 1 clock can drift by a second per day, sometimes much more. In addition, it was found that with iOS 4.2.1, clock syncing does not occur when connected to a Mac via iTunes.
The upshot is that doctors, astronomers, private pilots, researchers and others who depend on the clock in the Wi-Fi only iPad could find themselves many minutes off from the actual time, and that’s unacceptable for many purposes.
iPad 2 Update
My testing of a Wi-Fi only iPad 2 has shown that the situation is no better. With iOS 4.3.2, I found that syncing of the time still does not occur when connected to a Mac via iTunes and a full sync + backup is applied.
In addition, the oscillator that Apple has chosen for the iPad 2 doesn’t seem to be any better than that in the iPad 1. Over the past few weeks, I have been keeping a log of the time offset between an NTP time standard and the iPad 2’s internal clock. I did this by running an app from Emerald Sequoia called Emerald Time. It shows the offset, and then I would snap an iPad 2 screen shot to provide a photographic log — like this:
The iPad 2 offset (fast in this case) is shown in red: -20.549 sec.
I read the data off the screen shots and entered it into Apple’s Numbers spreadsheet, then created a chart. The data points have a linear, least squares fit applied. Here’s the chart of the drift.
Negative values in the chart indicate that the iPad’s internal clock is behind, positive numbers indicate that the iPad’s clock is running ahead of official time. This trend, from behind to ahead, is repeated on the iPad 2 from the iPad 1, that is, both clocks on my iPads run slightly fast. In the data above, the iPad 2 clock is gaining about 1.67 seconds/day which is almost a minute a month. (I switched the sign of the offsets to make the chart easier to read.)
Apple uses an oscillator in my iPad 2 that drifts just as much as the one in my iPad 1. The measured drift, in the iPad 2, is about 1.67 seconds/day. Nothing has changed in the iOS so that this Wi-Fi only iPad is time synced to a Mac when connected and synced. That would help a lot because Macs are typically connected to a time standard (by default time.apple.com) via NTP.
For users who must have good accuracy with their Wi-Fi only iPad 1 or 2, I strongly recommend Emerald Time, linked to above, and weekly resets of the iPad’s internal clock.