A number of iPad owners are reporting the devices sometimes will not keep the proper time. After a while of usage, the time will be off by anywhere from a few minutes to a few hours. This seems to happen randomly, where in some cases the device will be running fine for days on end and then suddenly show a time change.
This problem seems to happen when people use the power adaptor to charge the iPad when it is sleeping; however, some people suspect third-party clock and timing applications may be affecting the system clock, especially if they use a large amount of CPU or cause hangs. Removing and reinstalling the application may help.
One workaround for this issue that seems to help most people is to use the USB connector to charge the iPad from your Mac, and also frequently synchronize with your Mac. While it is unclear whether or not the iPad uses a time server to synchronize when connected to the internet, one or two people have found the device will automatically reset the time if WiFi is kept on.
Some people have found that the problem is not directly from the system going to sleep, but rather from the time not being updated after a lengthy sleep period. To get around this, when you wake the iPad from sleep just press sleep again, let it sit for a few minutes, and then wake it up again. The time should be toggled to update by this action.
Lastly, this problem could be from faults in the settings files for the iPad. While you cannot manage the files directly, you can try changing the time and time zone settings manually (toggle between a few settings) to see if that helps the situation. Although this has helped some users, others have found it to be only a temporary fix.
As a last resort, try backing up your iPad and then performing a full reset and restore of the system software. Running a fresh system on the relatively unmodifiable iPad hardware should have it running as expected. Test it this way for a while to ensure the bare software runs as it should, and then load your applications and other customizations to see if that affects anything. Hopefully the reset will clear faulty configurations that lead to this problem.